Clarify behavior of --no-cache-dir with respect to bdist_wheel

See pypa#5749

Author: ihnorton

news/E8975E25-403C-4022-899B-D192894358EA.trivial

@@ -576,7 +576,8 @@ def no_cache_dir_callback(option, opt, value, parser):
     dest="cache_dir",
     action="callback",
     callback=no_cache_dir_callback,
-    help="Disable the cache.",
+    help='Disable the cache. Warning: also disables the wheel '
+         'build step (see pip issue #5749).',
 )  # type: Callable[..., Option]
 
 no_deps = partial(