mm: improve __GFP_NORETRY comment based on implementation

Explicitly state that __GFP_NORETRY will attempt direct reclaim and
memory compaction before returning NULL and that the oom killer is not
called in the current implementation of the page allocator.

[akpm@linux-foundation.org: s/has/have/]
Signed-off-by: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/gfp.h

@@ -63,7 +63,10 @@ struct vm_area_struct;
  * but it is definitely preferable to use the flag rather than opencode endless
  * loop around allocator.
  *
- * __GFP_NORETRY: The VM implementation must not retry indefinitely.
+ * __GFP_NORETRY: The VM implementation must not retry indefinitely and will
+ * return NULL when direct reclaim and memory compaction have failed to allow
+ * the allocation to succeed.  The OOM killer is not called with the current
+ * implementation.
  *
  * __GFP_MOVABLE: Flag that this page will be movable by the page migration
  * mechanism or reclaimed