Clarify why we ENABLE_COPYWITHIN is false

kasmweb/core/websock.js

@@ -17,6 +17,8 @@ import * as Log from './util/logging.js';
 // this has performance issues in some versions Chromium, and
 // doesn't gain a tremendous amount of performance increase in Firefox
 // at the moment.  It may be valuable to turn it on in the future.
+// Also copyWithin() for TypedArrays is not supported in IE 11 or
+// Safari 13 (at the moment we want to support Safari 11).
 const ENABLE_COPYWITHIN = false;
 const MAX_RQ_GROW_SIZE = 40 * 1024 * 1024;  // 40 MiB