mm, doc: cleanup and clarify munmap behavior for hugetlb memory fix

mm, doc: cleanup and clarify munmap behavior for hugetlb memory fix

Don't only specify munmap(2) behavior with respect the hugetlb memory, all
other syscalls get naturally aligned to the native page size of the
processor.  Rather, pick out munmap(2) as a specific example.

Signed-off-by: David Rientjes <rientjes@google.com>
Acked-by: Hugh Dickins <hughd@google.com>
Acked-by: Hillf Danton <hillf.zj@alibaba-inc.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>