Rewrite constraints docs to be more helpful

juju/control/constraints_get.py

Index: juju/control/constraints_get.py
=== modified file 'juju/control/constraints_get.py'
--- juju/control/constraints_get.py	2012-03-30 09:41:54 +0000
+++ juju/control/constraints_get.py	2012-03-30 11:31:30 +0000
@@ -43,6 +43,8 @@
 
     This will show the final computed values of all constraints (including
     internal constraints which cannot be set directly via set-constraints).
+
+    If no entities are specified, environment constraints will be shown.
     """
     provider = environment.get_machine_provider()
     client = yield provider.connect()