page_alloc: fix kernel-doc warning

Ummark function as having kernel-doc notation, fixing the kernel-doc
warning.

Warning(mm/page_alloc.c:4519): No description found for parameter 'zone'

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>