Add mention to related system catalog functions on Tablespaces documentation pages.

The following documentation comment has been logged on the website:

Page: https://www.postgresql.org/docs/16/manage-ag-tablespaces.html
Description:

The tablespaces documentation page seems to be missing a mention to the
following functions:
pg_tablespace_databases
pg_tablespace_location

These are documented in
https://www.postgresql.org/docs/current/functions-info.html#FUNCTIONS-INFO-CATALOG,
but I think it would be nice to add at least a brief mention (or link) to
them in the main tablespace docs. The following query is especially helpful
when troubleshooting issues with tablespaces:

postgres=# SELECT *, pg_tablespace_location(oid) FROM pg_tablespace;

It would be beneficial, in my opinion, to use it as an additional example to
the one that is currently shown.

Best,
Agustín.