docs: event emitter behavior notice

Updated documentation as per the issue below:
https://github.com/joyent/node/issues/25466

Event listeners can alter parts of the passed object, in some
circumstances the changes are passed to the next listeners
due to pass by reference. This is documentation of that behavior.

PR-URL: https://github.com/joyent/node/pull/25467


Reviewed-By: jasnell - James M Snell <jasnell@gmail.com>