Add FAQ entry for retry subject; recipe w/ autocommit

Add FAQ entry for retry subject; recipe w/ autocommit

In order to invalidate a connection within a Transaction
block and an execution context, we need to take advantage
of the simpler transaction design added in 1.4.  The recipe
can be done on 1.3 but it requires a lot more hacking
and isn't worth it.

Clearly since the recipe is part of the tests now we can
in the future consider adding a feature that's built
in for this case but it would have to absolutely guarantee
the DBAPI is in autocommit mode and also prevent
any "write" operations from taking place.   Recipe for now.

Fixes: #5657
Change-Id: Ia9ea8cced084d154e83e4d1c259e080b776ec38a