docs/mm: add VMA locks documentation

Locking around VMAs is complicated and confusing.  While we have a number
of disparate comments scattered around the place, we seem to be reaching a
level of complexity that justifies a serious effort at clearly documenting
how locks are expected to be used when it comes to interacting with
mm_struct and vm_area_struct objects.

This is especially pertinent as regards the efforts to find sensible
abstractions for these fundamental objects in kernel rust code whose
compiler strictly requires some means of expressing these rules (and
through this expression, self-document these requirements as well as
enforce them).

The document limits scope to mmap and VMA locks and those that are
immediately adjacent and relevant to them - so additionally covers page
table locking as this is so very closely tied to VMA operations (and
relies upon us handling these correctly).

The document tries to cover some of the nastier and more confusing edge
cases and concerns especially around lock ordering and page table
teardown.

The document is split between generally useful information for users of mm
interfaces, and separately a section intended for mm kernel developers
providing a discussion around internal implementation details.

[lorenzo.stoakes@oracle.com: v3]
  Link: https://lkml.kernel.org/r/20241114205402.859737-1-lorenzo.stoakes@oracle.com
[lorenzo.stoakes@oracle.com: docs/mm: minor corrections]
  Link: https://lkml.kernel.org/r/d3de735a-25ae-4eb2-866c-a9624fe6f795@lucifer.local
[jannh@google.com: docs/mm: add more warnings around page table access]
  Link: https://lkml.kernel.org/r/20241118-vma-docs-addition1-onv3-v2-1-c9d5395b72ee@google.com
Link: https://lkml.kernel.org/r/20241108135708.48567-1-lorenzo.stoakes@oracle.com


Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Acked-by: Qi Zheng <zhengqi.arch@bytedance.com>
Acked-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>
Reviewed-by: Jann Horn <jannh@google.com>
Cc: Alice Ryhl <aliceryhl@google.com>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Hillf Danton <hdanton@sina.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Liam R. Howlett <Liam.Howlett@Oracle.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: SeongJae Park <sj@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>