mm/munlock: update Documentation/vm/unevictable-lru.rst

Update Documentation/vm/unevictable-lru.rst to reflect the changes made
by the mm/munlock series: keeping an mlock_count instead of page_mlock()
(formerly try_to_munlock()) and munlock_vma_pages_all() etc.  Also make
other little updates or cleanups wherever noticed.

But, I apologize, this is already out of date, in that "folio" appears
nowhere: 5.18 will be in a transitional state from "page" to "folio",
and documenting its current mix of the two does not help to understand
"the Unevictable LRU".  Should be revisited when naming is more settled.

Link: https://lkml.kernel.org/r/3753962-d491-bf60-f59f-51bfe84fd6a0@google.comSigned-off-by: Hugh Dickins <hughd@google.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: "Kirill A. Shutemov" <kirill@shutemov.name>
Cc: David Hildenbrand <david@redhat.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Rik van Riel <riel@surriel.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Yu Zhao <yuzhao@google.com>
Cc: Greg Thelen <gthelen@google.com>
Cc: Shakeel Butt <shakeelb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>