[fix][doc] Add client config override

[momo-jun]

Fixes #18490

Modifications

Add back the configuration override information introduced in #15818 but missed on the new Pulsar config site.

Documentation

• doc
• doc-required
• doc-not-needed
• doc-complete

[momo-jun]

With @tisonkun's help, I managed to add this chunk of information to the new Pulsar config page (in the Pulsar-site repo) and verify the changes. See apache/pulsar-site#294 for more details.

Maintaining the content in the new config page has pros/cons:

1. Pros: information is cohesive without a break.
2. Cons: not that easier for contributors/maintainers to update and verify, compared with the current config page.

Generally, I prefer to add this chunk to the new config page. If you don't have an objection, I will close this PR and continue with apache/pulsar-site#294.

cc @Anonymitaet @DaveDuggins @D-2-Ed

[Anonymitaet]

I'm +1 for apache/pulsar-site#294, reasons:

• All relevant info is in one place. Docs are not scattered around here and there. It's easier for both readers and maintainers.
• Explained in the image