Add "Understanding Application Transaction Editors" article to Diviner

Summary:
In writing a number of Phorge extensions, I found it quite challenging to
understand how to use and implement Application Transaction Editors. As these
are quite core to most Phorge applications, it seems that we should probably
have some documentation. Introduce a rough draft of a guide to using and
implementing such things.

Test Plan:
Check that the prose reads okay, and confirm that what's described is in line
with what PhabricatorApplicationTransactionEditor does. Additionally, run ./bin/diviner generate and observe that the document "Understanding Application Transaction Editors" appears in the appropriate location.

Reviewers: O1 Blessed Committers, valerio.bozzolan, aklapper

Reviewed By: O1 Blessed Committers, aklapper

Subscribers: aklapper, tobiaswiese, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D25883