Document classes and functions in core gcloud package

[dhermes]

H/T to @tseaver in #943 for suggesting / noticing

[dhermes]

Closing this out.

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It seems this was done long before in #542 (see gcloud-api.rst). In the original comment

TRES: Shouldn't we be documenting the "core" stuff somewhere else?
DAN: Yes we should (JSONConnection as well). Shall I do it here?

But JSONConnection was already documented.