Add message/client-summary endpoint docs

[vladvelici]

Description

Add client-summary endpoint docs. https://ably.atlassian.net/browse/CHA-1199

Replaces the old PR #2872 that was doing the same on an old base, in textile.

Checklist

• Commits have been rebased.
• Linting has been run against the changed file(s).
• The PR adheres to the writing style guide and contribution guide.

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[splindsay-92]

Added some small changes, but looks fine.