Add note that the XHR passed to XhrStream should not have had open() called on it already.
Add note that the XHR passed to XhrStream should not have had open() called on it already.

joshuabrown-ellation/libjass,Dador/libjass,joshuabrown-ellation/libjass,Dador/libjass,Arnavion/libjass,joshuabrown-ellation/libjass,Dador/libjass,Arnavion/libjass,Arnavion/libjass