Clarifications in user directory for users who share rooms tracking (#13966)

Spawned while working on [`get_users_in_room` mis-uses](https://github.com/matrix-org/synapse/pull/13958#discussion_r984074897) and thinking we could use `get_local_users_in_room` here but we can't.

From first glance, it seemed like this was only using local users from all of the `is_mine_id(user_id)` checks but I see that it does actually use remote users. Just making things a little more clear here what it does and mentions remote users so maybe that will be more obvious in the future.

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