Inaccurate usage of "which" instead of "that" in parallel.sgml

Hi everybody,

I noticed that in parallel.sgml, "which" is often used in defining 
clauses, where "that" is appropriate
(as explained at 

https://www.grammarly.com/blog/which-vs-that/?&utm_source=google&utm_medium=cpc&utm_campaign=11907689446&utm_targetid=aud-1117013793091:dsa-1233402314764&gclid=CjwKCAjwybyJBhBwEiwAvz4G77NN1eaRDOA4eGNa38UsoWXJwRG43pegXBNOiamA3Y0A3FHhZzFJJRoCXEMQAvD_BwE&gclsrc=aw.ds).

So I attached a patch to fix this and also to add a preceding comma 
before "which" in one non-defining clause.

Since the content of parallel.sgml is not new, the documentation for 
earlier versions of PostgreSQL has the same inaccuracies, and it would 
be great to fix them there too. The patch also applies to versions 12 
and 13, except one @@ section).

Thank you.

-- 
Elena Indrupskaya
Lead Technical Writer
Postgres Professional http://www.postgrespro.com 
<http://www.postgrespro.com>