Add note about completing successful auth before using QR code auth

[stuart938503]

Whilst testing QR code authentication recently, I ran into an issue where none of my test users could use QR code authentication. After some time, I found one could - seemingly randomly. After working with Microsoft on this issue via email, it was highlighted that the authentication methods for a user are cached and only refreshed after successful authentication using an existing method. As I had scoped QR code auth to these test users and then immediately tried to use it, authentication failed. Once I logged in using a different method first e.g. password, QR code auth then worked for subsequent authentications.

This PR adds a small note to the QR code auth documentation highlighting this subtle and crucial requirement.

[prmerger-automator]

@stuart938503 : 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 9f06acc:

✅ Validation status: passed

File	Status	Preview URL	Details
docs/identity/authentication/how-to-authentication-qr-code.md	✅Succeeded

For more details, please refer to the build report.

For any questions, please:

• Try searching the learn.microsoft.com contributor guides
• Post your question in the Learn support channel

[v-dirichards]

@AanjuSingh

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

[AanjuSingh]

#sign-off

[prmerger-automator]

@stuart938503 : Thanks for your contribution! The author(s) and reviewer(s) have been notified to review your proposed change.

[AanjuSingh]

@stuart938503: Does this issue occur when QR code policy is enabled, disabled, and then enabled again?

[v-dirichards]

@stuart938503: Does this issue occur when QR code policy is enabled, disabled, and then enabled again?

@AanjuSingh Would you like to hold off on merging to wait for @stuart938503's response, or should we proceed with reviewing and merging?

[v-dirichards]

#hold-off