mm/page_alloc.c: modify the comment section for alloc_contig_pages()

Clarify that the alloc_contig_pages() allocated range will always be
aligned to the requested nr_pages.

Link: https://lkml.kernel.org/r/1639545478-12160-1-git-send-email-anshuman.khandual@arm.com
Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: David Hildenbrand <david@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>