JSONField is 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.