[MDL-87117] Describe the new strings to improve activity descriptions

[sarjona]

Adds comprehensive documentation outlining the requirements and steps for third-party Activities and Resources to utilise the new help string structure (_summary, _help, and _tip) within the Activity Chooser introduced in MDL-87117. This allows plugins to offer the same rich, structured help as core modules.

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[Copilot]

Pull request overview

This PR adds documentation for the new Activity Chooser help string structure introduced in MDL-87117. The documentation explains how the new _summary, _help, and _tip language strings work to provide richer activity descriptions in Moodle 5.2.

Key changes:

• Documents the new optional modulename_summary language string for concise module introductions
• Describes the refined structure of the existing modulename_help string with Key features and Ways to use it sections
• Introduces the new optional modulename_tip language string for best practices and usage tips

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[laurentdavid]

Thanks @sarjona, clear and detailed enough. All good !

[aanabit]

The new documentation describes fine the changes on MDL-87117, so this PR seems ready to merge as soon as MDL-87117 is integrated in the next weekly.