mm, oom: add description of struct oom_control

Describe the purpose of struct oom_control and what each member does.

Also make gfp_mask and order const since they are never manipulated or
passed to functions that discard the qualifier.

Signed-off-by: David Rientjes <rientjes@google.com>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>