mediawiki.requestIdleCallback: Improve documentation

mediawiki.requestIdleCallback: Improve documentation

* Explain basic logic.
* Document the 'options.timeout' parameter of the native interface.
  The fallback shim ignores this parameter because it always executes
  callbacks using setTimeout(,1), which is before any timeout would
  expire. The native requestIdleCallback is smarter about scheduling
  callbacks at moments in time when no other priority things happen,
  at which point the timeout helps inform how long to wait at most
  for an idle period to naturally occur.

Change-Id: I45126b875547f64ec8b450a264b250260c41084c