mm/mempolicy: rewrite alloc_pages documentation

mainline inclusion
from mainline-5.13-rc1
commit 6421ec76
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I3ZVL2
CVE: NA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=6421ec764a62c51f810c5dc40cd45eeb15801ad9

-------------------------------------------------

Document alloc_pages() for both NUMA and non-NUMA cases as kernel-doc
doesn't care.

Link: https://lkml.kernel.org/r/20210225150642.2582252-6-willy@infradead.org


Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Acked-by: Mike Rapoport <rppt@linux.ibm.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
(cherry picked from commit 6421ec76)
Signed-off-by: Yongqiang Liu <liuyongqiang13@huawei.com>
Reviewed-by: Tong Tiangen <tongtiangen@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>