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.

cjpeg.c

@@ -5,7 +5,7 @@
  * Copyright (C) 1991-1998, Thomas G. Lane.
  * Modified 2003-2011 by Guido Vollbeding.
  * libjpeg-turbo Modifications:
- * Copyright (C) 2010, 2013-2014, 2017, D. R. Commander.
+ * Copyright (C) 2010, 2013-2014, 2017, 2020, D. R. Commander.
  * For conditions of distribution and use, see the accompanying README.ijg
  * file.
  *
@@ -179,15 +179,15 @@ usage(void)
   fprintf(stderr, "  -arithmetic    Use arithmetic coding\n");
 #endif
 #ifdef DCT_ISLOW_SUPPORTED
-  fprintf(stderr, "  -dct int       Use integer DCT method%s\n",
+  fprintf(stderr, "  -dct int       Use accurate integer DCT method%s\n",
           (JDCT_DEFAULT == JDCT_ISLOW ? " (default)" : ""));
 #endif
 #ifdef DCT_IFAST_SUPPORTED
-  fprintf(stderr, "  -dct fast      Use fast integer DCT (less accurate)%s\n",
+  fprintf(stderr, "  -dct fast      Use less accurate integer DCT method [legacy feature]%s\n",
           (JDCT_DEFAULT == JDCT_IFAST ? " (default)" : ""));
 #endif
 #ifdef DCT_FLOAT_SUPPORTED
-  fprintf(stderr, "  -dct float     Use floating-point DCT method%s\n",
+  fprintf(stderr, "  -dct float     Use floating-point DCT method [legacy feature]%s\n",
           (JDCT_DEFAULT == JDCT_FLOAT ? " (default)" : ""));
 #endif
   fprintf(stderr, "  -icc FILE      Embed ICC profile contained in FILE\n");