add the 'action' event to the documentation

[oliverlangan]

Looking at the code, it appears as though an 'action' event is emitted when an ACTION message is seen. However, the docs do not include action in the list of emitted events.

In testing, it appears as though this event does not quite parallel the other events:

• the last parameter is not the message object itself
• it can not be listened for using action# or action#channel