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>