docs/mm: expand vma doc to highlight pte freeing, non-vma traversal

docs/mm: expand vma doc to highlight pte freeing, non-vma traversal

The process addresses documentation already contains a great deal of
information about mmap/VMA locking and page table traversal and
manipulation.

However it waves it hands about non-VMA traversal.  Add a section for this
and explain the caveats around this kind of traversal.

Additionally, commit 6375e95f381e ("mm: pgtable: reclaim empty PTE page in
madvise(MADV_DONTNEED)") caused zapping to also free empty PTE page
tables.  Highlight this.

Link: https://lkml.kernel.org/r/20250604180308.137116-1-lorenzo.stoakes@oracle.com
Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>
Cc: Jann Horn <jannh@google.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Qi Zheng <zhengqi.arch@bytedance.com>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>