Cleanup HostingServiceClient documentation & API

Review Request #1578 — Created March 4, 2019 and updated

Review Board
The `HostingServiceClient` client class as largely undocumented: it
contained no argument or return type docs. This has been corrected.
Several of its methods also had mutable defaults, which has been
replaced with `None` so that the values do not persist to other calls.
The `_build_form_data` method has been made public and static, since it
should never have to access instance variables to do the serialization
of the provided data and is a generally useful method outside the class.

Testing Done:
- Ran unit tests.
- Built the docs and read through them.

Reviewed at