doc: recommend Infinity on emitter.setMaxListeners

Instead of recommending `0` as the magic value to set max listeners to
unlimited, recommend `Infinity`. This paves the way for `0` as a magic
value eventually being deprecated and finally removed.

PR-URL: #2559
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: targos - Michaël Zasso <mic.besace@gmail.com>
Reviewed-By: Brendan Ashworth <brendan.ashworth@me.com>
Reviewed-By: Roman Reiss <me@silverwind.io>

doc/api/events.markdown

@@ -92,7 +92,7 @@ Returns emitter, so calls can be chained.
 By default EventEmitters will print a warning if more than 10 listeners are
 added for a particular event. This is a useful default which helps finding
 memory leaks. Obviously not all Emitters should be limited to 10. This function
-allows that to be increased. Set to zero for unlimited.
+allows that to be increased. Set to `Infinity` (or `0`) for unlimited.
 
 Returns emitter, so calls can be chained.