Clean up documentation on secrets

[colebow]

Description

Fixing a few minor issues in the secrets documentation, as well as adding a sentence to clarify that environment variables can be contained within a scope.

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)

Docs

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

We're improving documentation.

Related issues, pull requests, and links

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

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( ) Release notes entries required with the following suggested text:

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

[mosabua]

I feel like we need to be even more explicit in the sense that the env variable value can be retrieved from a secret management system on the fly as part of the startup script or by the provisioning system..

and this enables a system where no secret is stored anywhere on the Trino cluster itself and everything is just in memory of the Trino startup scripts and JVM runtime..

Check with @jefflester on user facing wording that might help

[jefflester]

@mosabua I met with @colebow today to review this and I thought it looked good. Calling out the scope is the most important part to this which I think was achieved. Thumbs-up on my side 👍