Cleanup HostingServiceClient documentation & API
Cleanup HostingServiceClient documentation & API

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 https://reviews.reviewboard.org/r/8912/

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