Clarify the watch requisite docs

The paragraph which began underneath the example return data in the `watch` section ("If the 'changes' key contains a populated dictionary...") was actually factually inaccurate.

The rest of the section also needed some clarification, so I rewrote most of it.  I also added a note to tell people that `onchanges` is really what they're looking for in many cases.

Oh, and I made some of the headers lowercase.  Just because the requisites are in a title/header does not mean we should write them with incorrect case.

I would love a review by @thatch45, make sure that this is all factually accurate.  `watch` is confusing enough with correct docs.