chore: add a comment about how markdown stream dot pulse works

Author: gadenbuie

js/markdown-stream/markdown-stream.scss

@@ -66,6 +66,8 @@ pre:has(.code-copy-button) {
 }
 
 .markdown-stream-dot {
+  // The stream dot is appended with each streaming chunk update, so the pulse animation
+  // only shows up when streaming pauses but isn't complete.
   animation: markdown-stream-dot-pulse 2s infinite cubic-bezier(0.18, 0.89, 0.32, 1.28);
   display: inline-block;
   transform-origin: center;