Doc: Improve description of the "parallel_commit" option for postgres_fdw.

Doc: Improve description of the "parallel_commit" option for postgres_fdw.

It wasn't very clear that if this option was enabled, postgres_fdw would
commit remote transactions in parallel at main-transaction end, and
commit remote subtransactions in parallel at subtransaction end, due to
the references to (sub)transaction and other too specific documentation.
Clarify that.  Also reword to simplify.

Follow-up for commit 04e706d42.

Jonathan S. Katz, reviewed by Justin Pryzby, with some modifications by
me.

Discussion: https://postgr.es/m/1435bfd1-172b-de38-f590-4404a5a62eb0%40postgresql.org