man pages: document prefork process model

man pages: document prefork process model

Document the process model options -M

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13765

Signed-off-by: Gary Lockyer <gary@catalyst.net.nz>
Signed-off-by: Tim Beale <timbeale@catalyst.net.nz>
Reviewed-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Autobuild-User(master): Tim Beale <timbeale@samba.org>
Autobuild-Date(master): Thu Jan 31 04:25:14 CET 2019 on sn-devel-144

(manual merge of commit 4ecdcc4b462d03bec3ec36ba1b90f45d9977be8b,
 with description updated for v4.9 behaviour)