doc/dev/note: Clarify what rejecting a stream request entails

    (for example, the onion service `IncomingStreamRequestFilter`

    checks the current number of open streams on the circuit,

    and rejects the incoming stream request if it would cause the circuit

    to exceed the max allowed number of concurrent streams)

  * a `CmdChecker` to checks if the message

    is one of the allowed stream-opening messages for the circuit

    to exceed the max allowed number of concurrent streams).

    `IncomingStreamRequestFilter` returns an `IncomingStreamRequestDisposition`,

    which instructs the reactor to either accept the stream request, or to reject it,

    either by closing the circuit (`CloseCircuit`),

    or by sending a given `END` cell (`RejectRequest(End)`)

  * a `CmdChecker` (`IncomingCmdChecker` in `tor-proto`), which checks if the message

    is one of the allowed stream-opening messages for the circuit.

    If it is not, its `CmdChecker::check_msg()` function will return a

    `StreamProto` error, causing the circuit to be torn down.

  * if the stream request arrived on a hop

    that is not expecting stream requests, it will be rejected immediately

    (this is irrelevant for relays, because they only have a single hop,

    namely themselves)

    namely themselves), and the circuit will be closed with a protocol violation

    error

If the stream request passes these checks,

the reactor will create a new stream entry in its stream map