docs: add note about ngModel usage in structural directives guide (an…

…gular#29522)

When I tried one of the examples provided in the documention, encountered the " Can't bind to 'ngModel' since it isn't a known property of 'select' ” error. So to resolve it I imported the FormsModule in ngModule. If it is mentioned in the documentation, it would be handy for the beginners to try and learn. Thanks for considering.

Duplicate of pull Request# 29206, As I encountered few issues after multiple authors corrected, So I am creating the new pull request

PR Close angular#29522

aio/content/guide/structural-directives.md

@@ -590,6 +590,11 @@ The drop down works properly.
   <img src='generated/images/guide/structural-directives/select-ngcontainer-anim.gif' alt="ngcontainer options work properly">
 </figure>
 
+<div class="alert is-helpful">
+
+**Note:** Remember that ngModel directive is defined as a part of Angular FormsModule and you need to include FormsModule in the imports: [...] section of the Angular module metadata, in which you want to use it.
+
+</div>
 
 
 The `<ng-container>` is a syntax element recognized by the Angular parser.