Update comments for beginParsingChildren and finishParsingChildren

https://bugs.webkit.org/show_bug.cgi?id=268319

Reviewed by Anne van Kesteren.

Updated the comment to explain why relying on these functions should be avoided.

* Source/WebCore/dom/Element.h:

Canonical link: https://commits.webkit.org/273732@main

Source/WebCore/dom/Element.h

@@ -513,7 +513,8 @@ class Element : public ContainerNode {
     virtual void didStopBeingFullscreenElement() { }
 
     bool isFinishedParsingChildren() const { return isParsingChildrenFinished(); }
-    // Overriding these functions to make parsing a special case should be avoided if possible.
+    // Overriding these functions to make parsing a special case should be avoided if possible
+    // as that could lead to elements responding differently to the parser vs. other kinds of DOM mutations.
     void beginParsingChildren() { clearIsParsingChildrenFinished(); }
     virtual void finishParsingChildren();