Improve startup/shutdown in the admin guide

Fixes issue 877.
Signed-off-by: Jose A. Lopes <jabolopes@google.com>
Reviewed-by: Petr Pudlak <pudlak@google.com>

doc/admin.rst

@@ -358,6 +358,11 @@ after shutting down an instance, execute the following::
    on an instance Ganeti will automatically restart it (via
    the :command:`ganeti-watcher(8)` command which is launched via cron).
 
+Instances can also be shutdown by the user from within the instance, in
+which case they will marked accordingly and the
+:command:`ganeti-watcher(8)` will not restart them.  See
+:manpage:`gnt-cluster(8)` for details.
+
 Querying instances
 ~~~~~~~~~~~~~~~~~~