doc: update removeListener behaviour

This commit updates events doc to describe removeListener behaviour
when it is called within a listener. An example is added to make
it more evident.

A test is also incuded to make this behaviour consistent in future
releases.

Fixes: nodejs/node#4759

PR-URL: https://github.com/nodejs/node/pull/5201



Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>