Add knocking to the spec
Spec for https://github.com/matrix-org/matrix-doc/pull/2998
Spec for https://github.com/matrix-org/matrix-doc/pull/2403
This deliberately does not help towards fixing https://github.com/matrix-org/matrix-doc/issues/3153 in order to remain consistent with prior room versions, and to keep the diff smaller on this change. A future change will address room version legibility.