Rework Session transaction FAQs

In preparation for #4712, add an errors.rst code to the Session's
exception about waiting to be rolled back and rework the FAQ entry
to be much more succinct.  When this FAQ was first written, I found
it hard to describe why flush worked this way but as the use case is
clearer now, and #4712 actually showed it being confusing when it doesn't
work this way, we can make a simpler and more definitive statement
about this behavior.   Additionally, language about "subtransactions"
is minimized as I might be removing or de-emphasizing this concept in
2.0 (though maybe not as it does seem to work well).

Change-Id: I557872aff255b07e14dd843aa024e027a017afb8

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