Better patronictl help text for the "flush" subcommand (#1466)

Right now it's not really clear from --help what it does, flushing in software
context usually means persisting...so actually it's the opposite, so make
the intention more explicit.

patroni/ctl.py

@@ -868,7 +868,7 @@ def scaffold(obj, cluster_name, sysid):
     click.echo("Cluster {0} has been created successfully".format(cluster_name))
 
 
-@ctl.command('flush', help='Flush scheduled events')
+@ctl.command('flush', help='Discard scheduled events (restarts only currently)')
 @click.argument('cluster_name')
 @click.argument('member_names', nargs=-1)
 @click.argument('target', type=click.Choice(['restart']))