Add docs on changing ownership and touch upon writing custom fields.

Review Request #9310 — Created Oct. 23, 2017 and submitted — Latest diff uploaded


Review Board


This updates the Review Request Fields documentation to add a section on
the Submitter field, talking about its purpose and how in 3.0 the owner
can be reassigned.

It also adds a small section toward the bottom that mentions that the
list of fields can be changed by writing an extension, linking to the
new documentation for this.

Built the docs and read through them, checking for build errors and
broken links.