Update security docs for required shared secret

[mosabua]

Description

Only a MVP of an update .. we might have to revisit for more details, but I think this is critical to get in now since it is essentially a breaking change when authentication is enabled but no shared secret it configured.

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

Fix

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

Correction of docs.

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

Add fact that formerly optional config is now required when authentication is used.

Related issues, pull requests, and links

• Related documentation for Require internal shared secret is set when authentication is enabled #11944

Documentation

( ) No documentation is needed.
(✅ ) 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

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

[mosabua]

@dain @kokosing can you please review asap so that we can get this into 379 ... users like @ckesslau are having trouble with the update to 378 and stumble into this requirement.

cc @martint