docs($q): Document optional parameters for $q.when

This documents undocumented methods present in the `$q.when` API.
I don't really understand why these are said to be "alternate" --- the first one is a fulfilled promise handler, which is called when the promise returned by this function is fulfilled --- the second is the rejection handler, called if it's rejected, and the third is the notify handler, called if it receives notifications. Can you change the wording slightly to reflect those? thanks