pagewalk: add locking-rule comments

Originally, walk_hugetlb_range() didn't require a caller take any lock.
But commit d33b9f45 ("mm: hugetlb: fix hugepage memory leak in
walk_page_range") changed its rule.  Because it added find_vma() call in
walk_hugetlb_range().

Any locking-rule change commit should write a doc too.

[akpm@linux-foundation.org: clarify comment]
Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Cc: Hiroyuki Kamezawa <kamezawa.hiroyuki@gmail.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Matt Mackall <mpm@selenic.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>