docs: clear up default message persistence behavior for chat rooms

[splindsay-92]

Description

It was a bit unclear how persistence is managed for chat, so I've added a few lines to highlight how this works.

Checklist

• Commits have been rebased.
• Linting has been run against the changed file(s).
• The PR adheres to the writing style guide and contribution guide.

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[AndyTWF]

This para is probably too much detail for the docs, I think we can remove it and let the following two do the talking.

[AndyTWF]

@splindsay-92 I dont think this one is resolved? Im suggesting that we don't need to explain how the mechanism works here - too much info and for those interested is implied by the paragraphs above and below.

[splindsay-92]

@AndyTWF Sorry about that, I was sure I'd pushed a commit to remove this! dd82756