add limitation for AGIC

[fangzj1991]

add limitation for externalname service:
Azure/application-gateway-kubernetes-ingress#1567 (comment)

I notice several situations that user use externalname service as AGIC backend, although they can set hostname in AG manually but after AGIC pod restart it will revert and the configuration cannot load correctly.

[prmerger-automator]

@fangzj1991 : Thanks for your contribution! The author(s) and reviewer(s) have been notified to review your proposed change.

[learn-build-service-prod]

Learn Build status updates of commit 7c5ad4c:

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[v-regandowner]

@greg-lindsay - Can you review the proposed changes?

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

[prmerger-automator]

@fangzj1991 : Thanks for your contribution! The author(s) and reviewer(s) have been notified to review your proposed change.

[v-ccolin]

I sent an email to the content owner today.

@MicrosoftDocs/public-repo-pr-review-team

[v-dirichards]

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

Thanks!

[v-dirichards]

@greg-lindsay

Can you review this old PR and determine whether it needs to be closed or merged?

@MicrosoftDocs/public-repo-pr-review-team