(#3316) - more changes() documentation

The `changes()` documentation was looking kind of weak, so I added this.

Although I'm becoming convinced that people should just use `changes()` as an indication that *something* changed, rather than trying to understand the changes feed itself. E.g. if you are trying to maintain an in-memory mirror of DOM elements, then just blow it away every time something changes and run `allDocs` all over again. I believe I am even screwing this up in the ember-pouch plugin, so I need to go back and rewrite what I already wrote over there.