Clarify Private DNS support in Azure Firewall docs

[cpressland]

This PR updates the Azure Firewall DNS Settings documentation to clarify that while Azure Firewall can resolve Azure Private DNS zones for its own operations, the DNS Proxy feature does not return these records to downstream clients unless the configured upstream DNS server can also resolve them. This helps prevent confusion for users expecting Private DNS zones to be accessible via the DNS Proxy without additional configuration.

[cpressland]

@microsoft-github-policy-service agree

[prmerger-automator]

@cpressland : Thanks for your contribution! The author(s) and reviewer(s) have been notified to review your proposed change. @duau

[learn-build-service-prod]

Learn Build status updates of commit f4a060e:

✅ Validation status: passed

File	Status	Preview URL	Details
articles/firewall/dns-settings.md	✅Succeeded

For more details, please refer to the build report.

[ttorble]

@duongau

Can you review the proposed changes? Note that the author metadata should reflect your GitHub alias to facilitate automation. Can you create a PR in the private repo to update this? Thanks.

IMPORTANT: When the changes are ready for publication, adding a #sign-off comment is the best way to signal that the PR is ready for the review team to merge.

#label:"aq-pr-triaged"
@MicrosoftDocs/public-repo-pr-review-team

[v-dirichards]

@duongau Could you review this proposed update to your article and enter #sign-off in a comment if it's ready to merge?

Thanks!