Ensure that plpgsql cleans up cleanly during parallel-worker exit.

plpgsql_xact_cb ought to treat events XACT_EVENT_PARALLEL_COMMIT and
XACT_EVENT_PARALLEL_ABORT like XACT_EVENT_COMMIT and XACT_EVENT_ABORT
respectively, since its goal is to do process-local cleanup.  This
oversight caused plpgsql's end-of-transaction cleanup to not get done
in parallel workers.  Since a parallel worker will exit just after the
transaction cleanup, the effects of this are limited.  I couldn't find
any case in the core code with user-visible effects, but perhaps there
are some in extensions.  In any case it's wrong, so let's fix it before
it bites us not after.

In passing, add some comments around the handling of expression
evaluation resources in DO blocks.  There's no live bug there, but it's
quite unobvious what's happening; at least I thought so.  This isn't
related to the other issue, except that I found both things while poking
at expression-evaluation performance.

Back-patch the plpgsql_xact_cb fix to 9.5 where those event types
were introduced, and the DO-block commentary to v11 where DO blocks
gained the ability to issue COMMIT/ROLLBACK.

Discussion: https://postgr.es/m/10353.1585247879@sss.pgh.pa.us

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