mm/page-flags: fix comment

We haven't had 'dontuse' flags since 2002.  Replace this obsolete warning
with a hopefully more useful one.

Link: https://lkml.kernel.org/r/20201027025823.3704-1-willy@infradead.org
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: William Kucharski <william.kucharski@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/page-flags.h

@@ -86,8 +86,7 @@
  */
 
 /*
- * Don't use the *_dontuse flags.  Use the macros.  Otherwise you'll break
- * locked- and dirty-page accounting.
+ * Don't use the pageflags directly.  Use the PageFoo macros.
  *
  * The page flags field is split into two parts, the main flags area
  * which extends from the low bits upwards, and the fields area which