mm: document semantics of ZONE_MOVABLE

mm: document semantics of ZONE_MOVABLE

Let's document what ZONE_MOVABLE means, how it's used, and which special
cases we have regarding unmovable pages (memory offlining vs.  migration /
allocations).

Signed-off-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Mike Rapoport <rppt@linux.ibm.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Pankaj Gupta <pankaj.gupta.linux@gmail.com>
Cc: Baoquan He <bhe@redhat.com>
Cc: Jason Wang <jasowang@redhat.com>
Cc: Qian Cai <cai@lca.pw>
Link: http://lkml.kernel.org/r/20200816125333.7434-7-david@redhat.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>