Added context to B2C custom attribute requirements

[bolt-io]

ClaimTypes referencing an extension attribute start with extension_ but the reader is never informed to why that is. Now they will be.

[prmerger-automator[bot]]

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[Jak-MS]

@kengaderdus - Can you review this PR?

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

[prmerger-automator[bot]]

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

[prmerger-automator[bot]]

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

[kengaderdus]

#sign-off