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

Unintended functionality was removed from console.endTime by
#3562. Prior to that, you could
call console.endTime multiple times for the same label.

PR-URL: #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>

Author: ben-page

doc/api/console.md

@@ -230,6 +230,11 @@ console.timeEnd('100-elements');
 // prints 100-elements: 225.438ms
 ```
 
+*Note: As of Node.js v6.0.0, `console.timeEnd()` deletes the timer to avoid
+leaking it. On older versions, the timer persisted. This allowed
+`console.timeEnd()` to be called multiple times for the same label. This
+functionality was unintended and is no longer supported.*
+
 ### console.trace(message[, ...])
 
 Prints to `stderr` the string `'Trace :'`, followed by the [`util.format()`][]