Define 400 response for missing required headers

proposals/3886-simple-rendezvous-capability.md

@@ -71,6 +71,7 @@ HTTP request body:
 HTTP response codes, and Matrix error codes:
 
 - `201 Created` - rendezvous created
+- `400 Bad Request` (`M_MISSING_PARAM`) - no `Content-Length` was provided.
 - `403 Forbidden` (`M_FORBIDDEN`) - forbidden by server policy
 - `413 Payload Too Large` (`M_TOO_LARGE`) - the supplied payload is too large
 - `429 Too Many Requests` (`M_UNKNOWN`) - the request has been rate limited
@@ -114,6 +115,7 @@ HTTP request body:
 HTTP response codes, and Matrix error codes:
 
 - `202 Accepted` - payload updated
+- `400 Bad Request` (`M_MISSING_PARAM`) - no `Content-Length` was provided.
 - `404 Not Found` (`M_NOT_FOUND`) - rendezvous URI is not valid (it could have expired)
 - `412 Precondition Failed` (`M_DIRTY_WRITE`, **a new error code**) - when `If-Match` is supplied and the ETag does not match
 - `413 Payload Too Large` (`M_TOO_LARGE`) - the supplied payload is too large