doc: added note warning about change to console.endTime()

Unintended functionality was removed from console.endTime by
https://github.com/nodejs/node/pull/3562. Prior to that, you could
call console.endTime multiple times for the same label.

PR-URL: https://github.com/nodejs/node/pull/6454


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Robert Lindstaedt <robert.lindstaedt@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Jeremy Whitlock <jwhitlock@apache.org>