Document graceful_shutdown_timeout config option
Document graceful_shutdown_timeout config option

In change I0dfbf2d0a4943337da24c0904a1ed6f0cdccd77b
option graceful_shutdown_timeout was added.

Change-Id: I7564fc97dbe8fbbfc9f3b36e4dfbfabc078495e2

openstack/oslo.service