mm: make it clear that gfp reclaim modifiers are valid only for sleepable allocations

While it might be really clear to MM developers that gfp reclaim modifiers
are applicable only to sleepable allocations (those with
__GFP_DIRECT_RECLAIM) it seems that actual users of the API are not always
sure.  Make it explicit that they are not applicable for GFP_NOWAIT or
GFP_ATOMIC allocations which are the most commonly used non-sleepable
allocation masks.
Signed-off-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Acked-by: Paul E. McKenney <paulmck@kernel.org>
Acked-by: David Rientjes <rientjes@google.com>
Cc: Neil Brown <neilb@suse.de>
Link: http://lkml.kernel.org/r/20200403083543.11552-3-mhocko@kernel.orgSigned-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/gfp.h

@@ -124,6 +124,8 @@ struct vm_area_struct;
  *
  * Reclaim modifiers
  * ~~~~~~~~~~~~~~~~~
+ * Please note that all the following flags are only applicable to sleepable
+ * allocations (e.g. %GFP_NOWAIT and %GFP_ATOMIC will ignore them).
  *
  * %__GFP_IO can start physical IO.
  *