Add stable room summary -endpoint in worker docs.

[olmari]

Pull Request Checklist

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[olmari]

An courtesy check should be done for the endpoint, but I've synthesized the endpoint from the source https://github.com/element-hq/synapse/pull/19720/changes#diff-8a15d05534520d7c7e7f294d1646b3d2ae095a60a71c38ec57f6e9ee15ca10d3R1732 and it does match similar ones.