doc: correctly document the behavior of ee.once().

Addresses #5566. The `ee.once()` function is currently documented as
invoking the listener, and then removing it when the event is
triggered. However, this is not really the case. The listener is removed
and _then_ invoked. This only matters in a narrow set of use cases, but
when it matters, it matters that the docs are correct.

See the issue (#5566) for a discussion on why the code has not been
modified to match the documentation, but instead the documentation has
been modified to match the code.

Fixes: https://github.com/nodejs/node/issues/5566
PR-URL: https://github.com/nodejs/node/pull/6371


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Robert Lindstaedt <robert.lindstaedt@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>