Elaborate on terminate_connection documentation
Elaborate on terminate_connection documentation

We have had issues with drivers not handling terminate_connection
correctly when the connector argument is None. In these cases, the
driver is expected to terminate all connections to the volume.

This adds additional information to the volume driver interface
docstring to try to better point out this expectation.

Change-Id: Ibbf481da981428cd921fb9cb6bec6af8f6f330b3
Signed-off-by: Sean McGinnis <961eeb09b6d45516665e15b82e013282c4faf575@gmail.com>

openstack/cinder,mahak/cinder,mahak/cinder,openstack/cinder