Update MAUI Popup documentation for OnNavigatingFrom extension

[dartasen]

Related to CommunityToolkit/Maui#3099

[learn-build-service-prod]

PoliCheck Scan Report

The following report lists PoliCheck issues in PR files. Before you merge the PR, you must fix all severity-1 and severity-2 issues. The AI Review Details column lists suggestions for either removing or replacing the terms. If you find a false positive result, mention it in a PR comment and include this text: #policheck-false-positive. This feedback helps reduce false positives in future scans.

✅ No issues found

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[learn-build-service-prod]

Learn Build status updates of commit 46c5eaa:

✅ Validation status: passed

File	Status	Preview URL	Details
docs/maui/views/Popup.md	✅Succeeded	View

For more details, please refer to the build report.

[dartasen]

@microsoft-github-policy-service agree

[bijington]

@jfversluis you might want to make a u-turn... this functionality doesn't exist in the toolkit yet