doc: remove http2 pushStream weight option

The PRIORITY field is only supported by HTTP/2 in a HEADERS frame
initiated by the client, not in a push request sent by the server.

Documents the recommended approach to set a silent priority as
used internally by nghttp2.

PR-URL: https://github.com/nodejs/node/pull/16451


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>