docs(forms): make the forms validation guide specify more clearly what happens when mixing HTML5 native validators with Angular forms. (#42377)

Previously, the docs didn't say anything about the fact that the `novalidate` attribute is added to the enclosing form, or how to override that behavior. I have added a couple lines in the appropriate spot clarifying this issue.

PR Close #42377

aio/content/examples/reactive-forms/src/app/profile-editor/profile-editor.component.html

@@ -4,9 +4,7 @@
 <!-- #enddocregion ng-submit -->
   <label>
     First Name:
-    <!-- #docregion required-attribute -->
     <input type="text" formControlName="firstName" required>
-    <!-- #enddocregion required-attribute -->
   </label>
 
   <label>