docs/vm: add documentation of memory models

Describe what {FLAT,DISCONTIG,SPARSE}MEM are and how they manage to
maintain pfn <-> struct page correspondence.

Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/vm/index.rst

@@ -37,6 +37,7 @@ descriptions of data structures and algorithms.
    hwpoison
    hugetlbfs_reserv
    ksm
+   memory-model
    mmu_notifier
    numa
    overcommit-accounting