Specify how inclusive appservices interact

> Another question: if a join event for a room alias that should be forwarded to an app service comes in, and more than one app service's namespace matches it, should we GET `/rooms/{roomAlias}` on both app services, potentially creating two rooms? Because with the way things are currently set up, and alias namespaces aren't marked as exclusive, this could happen.
> API ref: https://matrix.org/docs/spec/application_service/unstable.html#get-rooms-roomalias

From @anoadragon453 in #dendrite-dev ( https://matrix.to/#/!FyQrGcOoVamcLcvdob:matrix.org/$15270053065148KAOsF:amorgan.xyz )

Related issue: https://github.com/matrix-org/matrix-doc/issues/559

Read the application service API reference linked in the issue and the related matrix-doc issue 559 first. Resolve how matching alias namespaces should be handled when multiple app services match, including whether both should receive GET /rooms/{roomAlias}; done means the specification states one unambiguous interaction.

Written by the indexing model from the issue text.