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: https://github.com/nodejs/node/pull/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>