Add UniqueKey documentation regarding const constructor

[shihaohong]

Description

Adds documentation about why a UniqueKey cannot have a const constructor.

Related Issues

Fixes #41043

Tests

n/a - doc update

Checklist

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Breaking Change

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[fluttergithubbot]

It looks like this pull request may not have tests. Please make sure to add tests before merging. If you need an exemption to this rule, contact Hixie.

Reviewers: Read the Tree Hygiene page and make sure this patch meets those guidelines before LGTMing.

[goderbauer]

LGTM