Improve HTMLScriptElement::didMoveToNewDocument() comment.

Source/core/html/HTMLScriptElement.cpp

Index: Source/core/html/HTMLScriptElement.cpp
diff --git a/Source/core/html/HTMLScriptElement.cpp b/Source/core/html/HTMLScriptElement.cpp
index bce71bdbcb9686e604a03408401a08950b6eeb5f..d429566d61384e6d8863194220a569b943ce31e2 100644
--- a/Source/core/html/HTMLScriptElement.cpp
+++ b/Source/core/html/HTMLScriptElement.cpp
@@ -75,12 +75,17 @@ void HTMLScriptElement::didMoveToNewDocument(Document& oldDocument)
 {
     RefPtrWillBeRawPtr<Document> contextDocument = document().contextDocument().get();
     if (!contextDocument) {
+        // Document's contextDocument() method will return no Document if the
+        // following conditions both hold:
+        //
+        //   - The Document wasn't created with an explicit context document
+        //     and that document is otherwise kept alive.
+        //   - The Document itself is detached from its frame.
+        //
+        // The script element's loader is in that case moved to document() and
+        // its script runner, which is the non-null Document that contextDocument()
+        // would return if not detached.
         ASSERT(!document().frame());
-        // A frame-detached document is handled as having no context
-        // document - it would be the document if not detached. The
-        // newly moved script element needs to be the latter here as
-        // the script loader for the pending script must also move to
-        // reside with that document and its script runner.
         contextDocument = &document();
     }
     oldDocument.scriptRunner()->movePendingAsyncScript(contextDocument->scriptRunner(), m_loader.get());