Clean up join pushdown docs for JDBC connectors

[jhlodin]

Description

With table statistics & cost-based join pushdowns added to select JDBC connectors, the join-pushdown.enabled property which previously defaulted to false on all JDBC connectors now defaults to true in these select connectors. This PR updates adds doc fragments that now accurately describe this property and its default value on a per-connector basis, as well as the needed title level and structure consistency changes needed to implement this improvement.

Is this change a fix, improvement, new feature, refactoring, or other?

Improvement

Is this a change to the core query engine, a connector, client library, or the SPI interfaces? (be specific)

Documentation

How would you describe this change to a non-technical end user or system administrator?

Make the join pushdown documentation consistent across connectors, while adding corrected information on how to configure join pushdown on JDBC connectors

Related issues, pull requests, and links

• Additional documentation for Add table statistics and automatic Join pushdown for PostgreSQL connector #11635, Add table statistics and automatic Join pushdown for SQL Server connector #11637, and Add table statistics and automatic Join pushdown for MySQL connector #11638
• Prepares documentation for Add table statistics, automatic Join pushdown, TopN pushdown for Oracle connector #11636

Documentation

( ) No documentation is needed.
(x) Sufficient documentation is included in this PR.
( ) Documentation PR is available with #prnumber.
( ) Documentation issue #issuenumber is filed, and can be handled later.

Release notes

(x) No release notes entries required.
( ) Release notes entries required with the following suggested text:

# Section
* Fix some things. ({issue}`issuenumber`)

[mosabua]

One minor correction .. the rest looks good.