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>