proc: fix documentation and description of pagemap

Since bit 57 was exported for uffd-wp write-protected (commit
fb8e37f3: "mm/pagemap: export uffd-wp protection information"),
fixing it can reduce some unnecessary confusion.

Link: https://lkml.kernel.org/r/20220301044538.3042713-1-yun.zhou@windriver.com
Fixes: fb8e37f3

 ("mm/pagemap: export uffd-wp protection information")
Signed-off-by: Yun Zhou <yun.zhou@windriver.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Tiberiu A Georgescu <tiberiu.georgescu@nutanix.com>
Cc: Florian Schmidt <florian.schmidt@nutanix.com>
Cc: Ivan Teterevkov <ivan.teterevkov@nutanix.com>
Cc: SeongJae Park <sj@kernel.org>
Cc: Yang Shi <shy828301@gmail.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Colin Cross <ccross@google.com>
Cc: Alistair Popple <apopple@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>