docs: clarify the behavior of `@SelectSnapshot`

[tnodet]

Update the documentation to clarify the fact that the @SelectSnapshot decorator sets up a property getter that will select the latest value on each access.

Closes #155

PR Checklist

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• [x] The commit message follows our guidelines: https://github.com/ngxs/store/blob/master/CONTRIBUTING.md#commit
• [ ] Tests for the changes have been added (for bug fixes / features)
• [ ] Docs have been added / updated (for bug fixes / features)

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[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[x] Documentation content changes
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Issue Number: #155