Rewrite runloop guide autorun FAQs

A rewrite of the autorun FAQs in the Runloop guide.

Fixes #1712 #1713 Partially addresses #1390
Agreed. _within_ is a poor choice. I was trying to get across that there was no magical scheduling of the callback code on the autorun. I'm not in love with the pseudocode example. I don't think it is particularly clear that the  'Ember.run.start()` seems quite magical out on its own. I will have another go at it but am happy to revert to your suggested edits if it doesn't pan out.