Add back offline user help for messagging

[Combs7th]

@claude, Let's update the Mattermost Product Documentation to reflect the end user messaging experience from Mattermost server v11.5 onward captured via engineering PR mattermost/mattermost#34756

Before proposing any changes:

• First, make an assessment on whether or not a doc update is even necessary for this PR.
• If it is necessary: Identify the existing end user documentation page(s) most closely related to messaging behavior and user presence states.
• Analyze the writing style, tone, structure, headings, and formatting patterns used on those pages.

When drafting updates:

• Match the existing voice and writing patterns of the page as closely as possible.
• Follow the same heading hierarchy and formatting conventions already in use.
• Avoid introducing new sections unless absolutely necessary.

Focus the docs update on:

• What has changed from the end user’s perspective when attempting to message an offline user
• Where this behavior appears in the product or UI
• When or why a user would encounter this help messaging
• Prioritize minimal documentation changes (this PR likely requires only a small clarification if any).
• Focus only on user-visible behavior, not implementation details.

DO NOT suggest changes to the following docs pages:

• changelogs
• important upgrade notes
• version archive
• removed and deprecated features
• unsupported legacy releases

[claude]

Claude finished @Combs7th's task in 2m 47s —— View job • claude/issue-8780-20260306-0041 • Create PR ➔

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Summary

• Gather context (read existing docs pages and engineering PR reference)
• Assess whether a doc update is necessary — Yes, it is
• Identify most relevant documentation pages
• Analyze writing style and conventions
• Draft and implement minimal documentation changes
• Commit and push changes

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Assessment: A documentation update is warranted. From Mattermost v11.5 (engineering PR #34756), Mattermost restores the help message shown when a user attempts to send a direct message to an Offline recipient — analogous to the existing DND and out-of-hours warnings already present in the docs.

Changes made (2 files, minimal):

1. source/end-user-guide/collaborate/send-messages.rst — Extended the notification sentence to include Offline alongside Do Not Disturb as a condition that triggers the warning above the message compose field.

2. source/end-user-guide/collaborate/channel-types.rst — Added a version-tagged sentence to the Direct message channels note stating that from v11.5, the same warning also appears when the recipient's availability is Offline.

Both changes follow the existing writing style, heading structure, and RST cross-reference conventions used on those pages.

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[Combs7th]

Done.