docs: clarify VS Code code review button visibility

[fhshaik]

Summary

Clarifies that the VS Code Copilot code review entry point applies when there are uncommitted changes to review.

Closes #44587.

Validation

• Ran git diff --check

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Table of review links

Note: Please update the URL for your staging server or codespace.

The table shows the files in the content directory that were changed in this pull request. This helps you review your changes on a staging server. Changes to the data directory are not included in this table.

Source	Review	Production	What Changed
copilot/how-tos/use-copilot-agents/request-a-code-review/use-code-review.md	fpt ghec	fpt ghec

Key: fpt: Free, Pro, Team; ghec: GitHub Enterprise Cloud; ghes: GitHub Enterprise Server

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