Clarify updatable properties for draft messages

[grtaylor806]

Updated properties to indicate they are updatable only if isDraft = true.

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• [] Link to PR for public-facing schema changes (schema-Prod-beta/v1.0.csdl): ADD LINK HERE
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[learn-build-service-prod]

Learn Build status updates of commit 2aaf533:

✅ Validation status: passed

File	Status	Preview URL	Details
api-reference/v1.0/api/message-update.md	✅Succeeded

For more details, please refer to the build report.

[FaithOmbongi]

See review feedback inline

[microsoft-github-policy-service]

Thank you for your work in this PR, @@grtaylor806.

When the updates in this PR are ready for external customers to use, please replace the do not merge label with ready to merge.

Check the content review workflow here.

[learn-build-service-prod]

Learn Build status updates of commit b09d8ab:

✅ Validation status: passed

File	Status	Preview URL	Details
api-reference/v1.0/api/message-update.md	✅Succeeded

For more details, please refer to the build report.