ec2_group: Document group_desc rule param

cloud/ec2_group

@@ -85,6 +85,11 @@ options:
     version_added: "1.6"
 
 requirements: [ "boto" ]
+
+notes:
+  - If a rule declares a group_name and that group doesn't exist, it will be
+    automatically created. In that case, group_desc should be provided as well.
+    The module will refuse to create a depended-on group without a description.
 '''
 
 EXAMPLES = '''
@@ -121,7 +126,9 @@ EXAMPLES = '''
       - proto: tcp
         from_port: 80
         to_port: 80
-        group_name: example
+        group_name: example-other
+        # description to use if example-other needs to be created
+        group_desc: other example EC2 group
 '''
 
 try: