vdsm: adding docstring to MultiProtocolAcceptor and ConnectedTcpServer

MultiProtocolAcceptor and ConnectedTCPServer now have docstring
explanation how they work.

Change-Id: Ie9bc02d93dfec10e95e4dd2554b797376f3a7543
Signed-off-by: pkliczewski <piotr.kliczewski@gmail.com>
Reviewed-on: http://gerrit.ovirt.org/28806
Reviewed-by: Dan Kenigsberg <danken@redhat.com>