documentation: update pid file description

The pid file behavior changed over time and the description is misleading.

http://tracker.ceph.com/issues/8324 Fixes: #8324

Signed-off-by: Loic Dachary <loic@dachary.org>
Merge pull request #1867 from dachary/wip-8324

documentation: update pid file description

Reviewed-by: Florent Flament <florent.flament@cloudwatt.com>