Update contained-availability-groups-overview.md

[ccarozzoMSFT]

Suggestion following a CSS engagement. Customer pushed his database on contained AG which blocked his production for some time before they understood they needed to recreate the logins. Adding explicit notification for this risk: "Once the user database has been joined to a contained AG, it will immediately become inaccessible to these users"

[prmerger-automator[bot]]

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

[learn-build-service-prod[bot]]

Learn Build status updates of commit b634c51:

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[Court72]

@MashaMSFT

Can you review the proposed changes?

When the changes are ready for publication, add a #sign-off comment to signal that the PR is ready for the review team to merge.

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

[MashaMSFT]

Thanks @ccarozzoMSFT ! #sign-off

[prmerger-automator[bot]]

PR 9797 has been merged from ccarozzoMSFT:patch-3 to MicrosoftDocs:live by Jak-MS.

@ccarozzoMSFT, @ccarozzo, @MashaMSFT