pgsql: Doc: Improve description of the "parallel_commit" option for pos

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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/8c8d307f82976122980bbccf940319d1b8a71403

Modified Files
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doc/src/sgml/postgres-fdw.sgml | 40 ++++++++++++++++++++++------------------
1 file changed, 22 insertions(+), 18 deletions(-)