do not show <instance> in help when not needed

base.py

@@ -83,7 +83,7 @@ class ServiceManager:
     pkg = ''
     revcmd = {}
     USAGE = None
-    main_ins = basename(expanduser('~'))
+    main_ins = None
 
     def __init__(self):
         self.env = {}
@@ -137,7 +137,7 @@ class ServiceManager:
         for ins in parser.sections():
             section = dict(parser[ins])
             if ins == 'MAIN':
-                ins = self.main_ins
+                ins = self.main_ins = basename(expanduser('~'))
             command = section.get('%s_command' % self.group)
             self.revcmd[command] = self.group
             nr = section.get(self.group)
@@ -410,5 +410,9 @@ class ServiceManager:
             raise
 
     def usage(self):
-        print(self.USAGE % ', '.join(self.info))
+        if self.main_ins:
+            usage = self.USAGE.replace(' <instance>', '').replace(' [<instance>]', '') % ''
+        else:
+            usage = self.USAGE % ('<instance> is one of %s' %  ', '.join(self.info))
+        print(usage)