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