docs(signals): improve SignalState docs

[markostanimirovic]

PR Checklist

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• [x] The commit message follows our guidelines: https://github.com/ngrx/platform/blob/main/CONTRIBUTING.md#commit
• [ ] Tests for the changes have been added (for bug fixes / features)
• [x] Documentation has been added / updated (for bug fixes / features)

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What kind of change does this PR introduce?

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
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[x] No

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