Update message-update.md to specify updatable conditions

[grtaylor806]

Clarified that certain properties are only updatable if isDraft is true.

Important

Required for API changes:

• [] Link to API.md file: ADD LINK HERE
• [] Link to PR for public-facing schema changes (schema-Prod-beta/v1.0.csdl): ADD LINK HERE
• [] Attach the documentation plan generated for AI-assisted docs authoring and validation to help speed up content review.

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Add other supporting information, such as a description of the PR changes:

ADD INFORMATION HERE

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[learn-build-service-prod]

Learn Build status updates of commit ca672cd:

✅ Validation status: passed

File	Status	Preview URL	Details
api-reference/beta/api/message-update.md	✅Succeeded

For more details, please refer to the build report.

[learn-build-service-prod]

Learn Build status updates of commit 283440e:

✅ Validation status: passed

File	Status	Preview URL	Details
api-reference/beta/api/message-update.md	✅Succeeded

For more details, please refer to the build report.