doc: Explicitly indicate that EventEmitter's listeners will still be invoked if they were removed during the emit cycle

[drjokepu]

As discussed in nodejs/node-v0.x-archive#7872, EventEmitter's listeners are still called if they are removed during the emit() cycle. It was agreed that this is the intended behavior, however, there is nothing in EventEmitter's documentation that would explicitly indicate that.

I think the documentation needs to be updated to clarify this behavior, because it's not necessarily obvious.

[cjihrig]

Documentation patches are welcome.