postgres_fdw "parallel_commit" docs

Hi,

While researching PG15 features, I was trying to read through the
docs[1] for the "parallel_commit" (04e706d4) feature in postgres_fdw to
better understand what it does. I found myself becoming lost with the
references to (sub)transaction and a few other items that, while
accurate, may be overly specific in this context.

Attached is a patch to try to simplify the language for the description
of the "parallel_commit" option. A few notes:

* I stated that this feature applies to both transactions and
subtransactions.
* I tried to condense some of the language around remote/local
transactions. If this makes the statement inaccurate, let's revise.
* I removed the "Be careful with this option" and instead clarified an
explanation of the case that could cause performance impacts.

This feature seems like it will be impactful for distributed workloads
using "postgres_fdw" so I want to ensure that we both accurately and
clearly capture what it can do.

Thanks!

Jonathan

[1]
https://www.postgresql.org/docs/devel/postgres-fdw.html#id-1.11.7.47.11.7