Update how-to-managed-attributes.md

[evangelinew]

Updates known limitations to say managed attributes can be deleted but only if they have no references.

[prmerger-automator[bot]]

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

[evangelinew]

#sign-off

[prmerger-automator[bot]]

Invalid command: '#sign-off'. Only the assigned author of one or more file in this PR can sign off. @nayenama

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

Learn Build status updates of commit d47ce0d:

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[Jak-MS]

@nayenama

• Can you review this PR?
• IMPORTANT: When this content is ready to merge, you must add #sign-off in a comment or the approval may get overlooked.

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

[whhender]

@evangelinew - you're just in azure-docs, not azure-docs-pr. If you close this and open one in azure-docs-pr you'll be able to sign off on it.

[Jak-MS]

@whhender - do you approve this? we'll accept your sign off here. thanks

[Jak-MS]

@whhender - ignore the last comment; i see your approval in email. merging.