Document configuration on how to shard Synapse's logical databases

This issue has been migrated from [#11491](https://github.com/matrix-org/synapse/issues/11491).

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One can do so via the `databases` top-level config option, replacing the `database` one. This feature is still experimental, and should be classified as such (we haven't yet tried it on matrix.org for instance):

https://github.com/matrix-org/synapse/blob/ba5287f5e8be150551824493b3ad685dde00a543/synapse/config/database.py#L123-L136

We currently document this feature here (without mentioning that it is experimental): https://matrix-org.github.io/synapse/latest/development/database_schema.html?highlight=state_gro#logical-databases

We may want to hold off adding documentation about it to the sample config until it is stable, but thoughts welcome.