We have a few commands that "can't run in a transaction block",
meaning that if they complete their processing but then we fail
to COMMIT, we'll be left with inconsistent on-disk state.
However, the existing defenses for this are only watertight for
simple query protocol.  In extended protocol, we didn't commit
until receiving a Sync message.  Since the client is allowed to
issue another command instead of Sync, we're in trouble if that
command fails or is an explicit ROLLBACK.  In any case, sitting
in an inconsistent state while waiting for a client message
that might not come seems pretty risky.

This case wasn't reachable via libpq before we introduced pipeline
mode, but it's always been an intended aspect of extended query
protocol, and likely there are other clients that could reach it
before.

To fix, set a flag in PreventInTransactionBlock that tells
exec_execute_message to force an immediate commit.  This seems
to be the approach that does least damage to existing working
cases while still preventing the undesirable outcomes.

While here, add some documentation to protocol.sgml that explicitly
says how to use pipelining.  That's latent in the existing docs if
you know what to look for, but it's better to spell it out; and it
provides a place to document this new behavior.

Per bug #17434 from Yugo Nagata.  It's been wrong for ages,
so back-patch to all supported branches.

Discussion: https://postgr.es/m/17434-d9f7a064ce2a88a3@postgresql.org