clarify ManualEventAgent usage

app/models/agents/manual_event_agent.rb

@@ -5,6 +5,9 @@ module Agents
 
     description <<-MD
       The Manual Event Agent is used to manually create Events for testing or other purposes.
+
+      Do not set options for this Agent.  Instead, connect it to other Agents and create Events
+      using the UI provided on this Agent's Summary page.
     MD
 
     event_description "User determined"
@@ -29,4 +32,4 @@ module Agents
     def validate_options
     end
   end
-end
+end