We probably shouldn't be sending App Services events from every room we know about

Nobody has claimed this yet.

While undocumented in the [Application Services API](https://matrix.org/docs/spec/application_service/unstable.html#registration), Synapse's current behavior is to check on any event it sees for whether an app service is interested in it, and then send it off to that app service. We should further filter these by the rooms than an application service are a part of.

Without this constraint, for instance with Half-Shot's [Discord Bridge](https://github.com/half-shot/matrix-appservice-discord), typing `!discord` in any channel causes about 10 different bots running on 10 different homeservers to try and answer. This also reportedly affects @turt2live's webhook bridge.

I plan to build the current loose functionality into Dendrite just to remain backwards compatible, but we really should change this unless anyone can give a reason why app services should get events from rooms they aren't in.

Read the Application Services API registration section and compare its documented behavior with Synapse's current event filtering and the compatibility concern raised for Dendrite. Determine whether room membership should constrain delivery, then update the relevant specification text and add any required examples or conformance guidance.

Written by the indexing model from the issue text.