This writes up matrix-org/matrix-spec-proposals#3981

Hopefully this is relatively straightforward, apart from having to add
the parameters and response field in all three places. I tried to factor
these out but it seems references just aren't supported in the right
places currently (see #1745
for my efforts). Path parameters can't be optional, so it can't be done
that way either.

Co-authored-by: Richard van der Hoff <[email protected]>

Co-authored-by: Richard van der Hoff <[email protected]>

Co-authored-by: Richard van der Hoff <[email protected]>

Co-authored-by: Richard van der Hoff <[email protected]>

Co-authored-by: Richard van der Hoff <[email protected]>

Note this is counter what the MSC actually proposed to add, but
I think it's clear that this is what it meant.