Add comment for new div

Author: adolgachev

src/material/form-field/form-field.html

@@ -101,6 +101,11 @@
 >
   @let subscriptMessageType = _getSubscriptMessageType();
 
+  <!-- 
+    Use a single permanent wrapper for both hints and errors so aria-live works correctly,
+    as having it appear post render will not consistently work. We also do not want to add
+    additional divs as it causes styling regressions.
+    -->
   <div aria-atomic="true" aria-live="polite" 
       [class.mat-mdc-form-field-error-wrapper]="subscriptMessageType === 'error'"
       [class.mat-mdc-form-field-hint-wrapper]="subscriptMessageType === 'hint'"