Clarify updatable properties for draft messages

[grtaylor806]

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

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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

[FaithOmbongi]

@grtaylor806 - not: Please format all instances to match this change to ensure the formatted text remains the same in translated content. Update the beta file as well. Thank you.

Suggested change

	|bccRecipients|Recipient|The Bcc recipients for the message. Updatable only if isDraft = true.|
	|bccRecipients|Recipient|The Bcc recipients for the message. Updatable only if **isDraft** = `true`.|

[grtaylor806]

@FaithOmbongi - I was matching the style of other lines that already have this comment/statement, in that style. Lines 47 and 59 for example. So are you saying we need to change all of those lines too?