The API code was written before our current documentation standards, and

largely lacked any documentation. Some of the documentation we had was

incomplete or misleading. This made it a bit hard to reason about this

part of the codebase and make changes to it, and made it hard to consume

without reading through a lot of complex code.
This change fleshes out documentation for several of the major

components in the communication layers of the API, bringing these parts

up-to-date with our standards.
To help with readability, it also fixes up a few function signatures in

places, marks a few internal attributes in ReviewBoardServer as

private, and removes some unneeded attributes in

ReviewBoardHTTPPasswordMgr.

Posted this change successfully.
Unit tests pass.