@MSC2666: Add "may return 400" qualifier (#3770)
Resolves [this comment](https://github.com/matrix-org/matrix-spec-proposals/pull/2666#discussion_r830500133)
Rationale for `M_UNKNOWN`: There's not a "semantically more close" variant of that error, and i'm hesitant to add a new error.
cc @Half-Shot
