Demote "fast" [I]DCT algorithms to legacy status

- Refer to the "slow" [I]DCT algorithms as "accurate" instead, since
  they are not slow under libjpeg-turbo.
- Adjust documentation claims to reflect the fact that the "slow" and
  "fast" algorithms produce about the same performance on AVX2-equipped
  CPUs (because of the dual-lane nature of AVX2, it was not possible to
  accelerate the "fast" algorithm beyond what was achievable with SSE2.)
  Also adjust the claims to reflect the fact that the "fast" algorithm
  tends to be ~5-15% faster than the "slow" algorithm on
  non-AVX2-equipped CPUs, regardless of the use of the libjpeg-turbo
  SIMD extensions.
- Indicate the legacy status of the "fast" and float algorithms in the
  documentation and cjpeg/djpeg usage info.
- Remove obsolete paragraph in the djpeg man page that suggested that
  the float algorithm could be faster than the "fast" algorithm on some
  CPUs.

jpeglib.h

@@ -5,7 +5,7 @@
  * Copyright (C) 1991-1998, Thomas G. Lane.
  * Modified 2002-2009 by Guido Vollbeding.
  * libjpeg-turbo Modifications:
- * Copyright (C) 2009-2011, 2013-2014, 2016-2017, D. R. Commander.
+ * Copyright (C) 2009-2011, 2013-2014, 2016-2017, 2020, D. R. Commander.
  * Copyright (C) 2015, Google, Inc.
  * For conditions of distribution and use, see the accompanying README.ijg
  * file.
@@ -244,9 +244,9 @@ typedef enum {
 /* DCT/IDCT algorithm options. */
 
 typedef enum {
-  JDCT_ISLOW,             /* slow but accurate integer algorithm */
-  JDCT_IFAST,             /* faster, less accurate integer method */
-  JDCT_FLOAT              /* floating-point: accurate, fast on fast HW */
+  JDCT_ISLOW,             /* accurate integer method */
+  JDCT_IFAST,             /* less accurate integer method [legacy feature] */
+  JDCT_FLOAT              /* floating-point method [legacy feature] */
 } J_DCT_METHOD;
 
 #ifndef JDCT_DEFAULT            /* may be overridden in jconfig.h */