IJG dox: Wordsmithing and formatting tweaks

- Remove the section in libjpeg.txt that advised against building
  libjpeg as a shared library.  We obviously do not follow that advice,
  and libjpeg-turbo does guarantee backward ABI compatibility in our
  libjpeg API library, even though libjpeg did not and does not.
  (Future expansion of our libjpeg API library, if necessary, will be
  accomplished using get/set functions that store the new parameters
  in the opaque master structs.  Refer to
  db2986c96f.)

- Unmention install.txt, which was never relevant to libjpeg-turbo and
  was removed in v1.3 (6f96153c67).

- Remove extraneous spaces.

- Document the fact that TWO_FILE_COMMANDLINE must be defined in order
  to use the two-file interface with cjpeg, djpeg, jpegtran, and
  wrjpgcom.  libjpeg-turbo never enables that interface by default.

libjpeg.txt

@@ -3,7 +3,7 @@ USING THE IJG JPEG LIBRARY
 This file was part of the Independent JPEG Group's software:
 Copyright (C) 1994-2013, Thomas G. Lane, Guido Vollbeding.
 libjpeg-turbo Modifications:
-Copyright (C) 2010, 2014-2018, 2020, D. R. Commander.
+Copyright (C) 2010, 2014-2018, 2020, 2022, D. R. Commander.
 Copyright (C) 2015, Google, Inc.
 For conditions of distribution and use, see the accompanying README.ijg file.
 
@@ -840,18 +840,7 @@ is to prepare a library file ("libjpeg.a", or a corresponding name on non-Unix
 machines) and reference it at your link step.  If you use only half of the
 library (only compression or only decompression), only that much code will be
 included from the library, unless your linker is hopelessly brain-damaged.
-The supplied makefiles build libjpeg.a automatically (see install.txt).
-
-While you can build the JPEG library as a shared library if the whim strikes
-you, we don't really recommend it.  The trouble with shared libraries is that
-at some point you'll probably try to substitute a new version of the library
-without recompiling the calling applications.  That generally doesn't work
-because the parameter struct declarations usually change with each new
-version.  In other words, the library's API is *not* guaranteed binary
-compatible across versions; we only try to ensure source-code compatibility.
-(In hindsight, it might have been smarter to hide the parameter structs from
-applications and introduce a ton of access functions instead.  Too late now,
-however.)
+The supplied build system builds libjpeg.a automatically.
 
 It may be worth pointing out that the core JPEG library does not actually
 require the stdio library: only the default source/destination managers and
@@ -3075,9 +3064,8 @@ BITS_IN_JSAMPLE as 12 rather than 8.  Note that this causes JSAMPLE to be
 larger than a char, so it affects the surrounding application's image data.
 The sample applications cjpeg and djpeg can support 12-bit mode only for PPM
 and GIF file formats; you must disable the other file formats to compile a
-12-bit cjpeg or djpeg.  (install.txt has more information about that.)
-At present, a 12-bit library can handle *only* 12-bit images, not both
-precisions.
+12-bit cjpeg or djpeg.  At present, a 12-bit library can handle *only* 12-bit
+images, not both precisions.
 
 Note that a 12-bit library always compresses in Huffman optimization mode,
 in order to generate valid Huffman tables.  This is necessary because our

usage.txt

@@ -25,7 +25,7 @@ GENERAL USAGE
 We provide two programs, cjpeg to compress an image file into JPEG format,
 and djpeg to decompress a JPEG file back into a conventional image format.
 
-On Unix-like systems, you say:
+On most systems, you say:
         cjpeg [switches] [imagefile] >jpegfile
 or
         djpeg [switches] [jpegfile]  >imagefile
@@ -34,19 +34,19 @@ named.  They always write to standard output (with trace/error messages to
 standard error).  These conventions are handy for piping images between
 programs.
 
-On most non-Unix systems, you say:
+If you defined TWO_FILE_COMMANDLINE when compiling the programs, you can
+instead say:
         cjpeg [switches] imagefile jpegfile
 or
         djpeg [switches] jpegfile  imagefile
 i.e., both the input and output files are named on the command line.  This
 style is a little more foolproof, and it loses no functionality if you don't
-have pipes.  (You can get this style on Unix too, if you prefer, by defining
-TWO_FILE_COMMANDLINE when you compile the programs; see install.txt.)
+have pipes.
 
 You can also say:
         cjpeg [switches] -outfile jpegfile  imagefile
 or
-        djpeg [switches] -outfile imagefile  jpegfile
+        djpeg [switches] -outfile imagefile jpegfile
 This syntax works on all systems, so it is useful for scripts.
 
 The currently supported image file formats are: PPM (PBMPLUS color format),
@@ -143,8 +143,8 @@ customized) quantization tables can be set with the -qtables option and
 assigned to components with the -qslots option (see the "wizard" switches
 below.)
 
-JPEG  files  generated  with separate luminance and chrominance quality are
-fully compliant with standard JPEG decoders.
+JPEG files generated with separate luminance and chrominance quality are fully
+compliant with standard JPEG decoders.
 
 CAUTION: For this setting to be useful, be sure to pass an argument of
 -sample 1x1 to cjpeg to disable chrominance subsampling.  Otherwise, the
@@ -218,7 +218,7 @@ Switches for advanced users:
                         space is needed, an error will occur.
 
         -verbose        Enable debug printout.  More -v's give more printout.
-        or  -debug      Also, version information is printed at startup.
+        or -debug       Also, version information is printed at startup.
 
 The -restart option inserts extra markers that allow a JPEG decoder to
 resynchronize after a transmission error.  Without restart markers, any damage
@@ -467,9 +467,10 @@ quality.  However, while the image data is losslessly transformed, metadata
 can be removed.  See the -copy option for specifics.
 
 jpegtran uses a command line syntax similar to cjpeg or djpeg.
-On Unix-like systems, you say:
+On most systems, you say:
         jpegtran [switches] [inputfile] >outputfile
-On most non-Unix systems, you say:
+If you defined TWO_FILE_COMMANDLINE when compiling the program, you can instead
+say:
         jpegtran [switches] inputfile outputfile
 where both the input and output files are JPEG files.
 
@@ -649,13 +650,13 @@ file; it does not modify the input file.  DO NOT try to overwrite the input
 file by directing wrjpgcom's output back into it; on most systems this will
 just destroy your file.
 
-The command line syntax for wrjpgcom is similar to cjpeg's.  On Unix-like
-systems, it is
+The command line syntax for wrjpgcom is similar to cjpeg's.  On most systems,
+it is
         wrjpgcom [switches] [inputfilename]
 The output file is written to standard output.  The input file comes from
 the named file, or from standard input if no input file is named.
 
-On most non-Unix systems, the syntax is
+If you defined TWO_FILE_COMMANDLINE when compiling the program, the syntax is:
         wrjpgcom [switches] inputfilename outputfilename
 where both input and output file names must be given explicitly.