Improve documentation and clean up code for JSONField.
Review Request #9406 - Created Nov. 28, 2017 and submitted
JSONFieldis one of the older classes in Djblets, and had very little
documentation available, despite all the work done on it over the years.
This change fixes that up by adding documentation for all methods and
attributes using our standard doc format.
Some of the code in the class was also cleaned up a bit, helping remove
some redundant code and bad/stale comments.
Unit tests passed.
Tested manually manipulating a JSON field through the admin UI to make
sure nothing broke anywhere.