documenting  that on() handler accepts .trigger() args #472

See #472 for the complete story. Basically;

 1. Added `extraParameter` to the signature, and added further documentation as to how it works. It uses the `rest="true"` vararg attribute, which currently doesn't *show* anything, but support is being added in https://github.com/jquery/grunt-jquery-content/issues/56
 2. Added examples of using `on()` and `trigger()`.
 3. Various tidyings

... all separated into separate commits.
Humm. I consciously used that variable name so that it matched the `extraParameter` variable name in the signature definition at the top of the `on()` docs; that way it'd be a CTRL + F match for anyone wondering WTF the `extraParameter` variable was in the signature (as otherwise it wouldn't be searchable).

I'm not sure whether you'd prefer CTRL + F goodness over good variable names; I'm open to change it.