Require `PUT` to supply an `If-Match`

proposals/3886-simple-rendezvous-capability.md

@@ -109,7 +109,8 @@ HTTP request headers:
 
 - `Content-Length` - required
 - `Content-Type` - optional, server should assume `application/octet-stream` if not specified
-- `If-Match` - optional, as per [RFC7232](https://httpwg.org/specs/rfc7232.html#header.if-match) server will assume `*`
+- `If-Match` - required. The ETag of the last payload seen by the requesting device.
+
 if not specified
 
 HTTP request body:
@@ -119,9 +120,9 @@ 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.
+- `400 Bad Request` (`M_MISSING_PARAM`) - a required header was not 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
+- `412 Precondition Failed` (`M_DIRTY_WRITE`, **a new error code**) - when the ETag does not match
 - `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