/joined_members makes implementation specific claims about speed and use cases

https://matrix.org/docs/spec/client_server/r0.5.0#get-matrix-client-r0-rooms-roomid-joined-members

This is very much a nit, but given we are enforcing new proposals not to make such claims in the spec, we shouldn't make such claims in the current spec.

>This API is primarily for Application Services and should be faster to respond than /members as it can be implemented more efficiently on the server.

The API shouldn't be described as *primarily* for app services because it gives the impression it is not useful for clients or bots, which of course it's perfectly fine to use.

The API certainly shouldn't make claims about speed, because it might well be the case that somebody's amazing Rust homeserver can be equally fast for /members, and the fact given here in the spec is based off a the reference implementation's speed issues.

Open the linked Matrix client-server specification section for GET /_matrix/client/r0/rooms/{roomId}/joined_members and review the quoted description. Remove the implementation-specific speed claim and the suggestion that the API is primarily for application services; done means the section describes the API without those claims.

Written by the indexing model from the issue text.