Rename and Document mongo and xblock call counting

Description

It would be good to make `check_mongo_calls` and an alias that counts XBlock instantiations part of a TestCase subclass (as per the comment here: https://github.com/edx/edx-platform/pull/8977#discussion-diff-35020775). Ideally, we would also be able to use nested versions of the decorators with the bulk assertion manager, but it's not clear that that can happen (or, rather, how to make that happen).

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Reporter

Calen Pennington

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