doc: use consistent event name parameter

Implementing the suggestion in
https://github.com/nodejs/node/issues/4554 this pull request renames
the parameter name in all the places that accept an event name as a parameter.

Previously, the parameter has been called `event` or `type`. Now as suggested
it is consistently called `eventName`.

PR-URL: https://github.com/nodejs/node/pull/5850


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>