docs: Documents the disableRtx config option.

config.js

@@ -79,5 +79,6 @@ var config = { // eslint-disable-line no-unused-vars
     enableUserRolesBasedOnToken: false,
     // Suspending video might cause problems with audio playback. Disabling until these are fixed.
     disableSuspendVideo: true,
+    // disables or enables RTX (RFC 4588).
     disableRtx: true
 };