docs: warn that directives don't support namespaces (#25855)

PR Close #25855

aio/content/examples/attribute-directives/src/app/app.component.1.html

@@ -2,7 +2,7 @@
 <h1>My First Attribute Directive</h1>
 <!-- #docregion applied -->
 <p appHighlight>Highlight me!</p>
-<!-- #enddocregion applied,  -->
+<!-- #enddocregion applied  -->
 
 <!-- #docregion color-1 -->
 <p appHighlight highlightColor="yellow">Highlighted in yellow</p>
@@ -11,4 +11,4 @@
 
 <!-- #docregion color-2 -->
 <p appHighlight [highlightColor]="color">Highlighted with parent component's color</p>
-<!-- #enddocregion color-2 -->
+<!-- #enddocregion color-2 -->

aio/content/examples/attribute-directives/src/app/app.component.avoid.html

@@ -0,0 +1,3 @@
+<!-- #docregion unsupported -->
+<p app:Highlight>This is invalid</p>
+<!-- #enddocregion unsupported  -->

aio/content/guide/attribute-directives.md

@@ -41,6 +41,10 @@ when the user hovers over that element. You can apply it like this:
 
 {@a write-directive}
 
+Please note that directives _do not_ support namespaces.
+
+<code-example path="attribute-directives/src/app/app.component.avoid.html" linenums="false" header="src/app/app.component.avoid.html (unsupported)" region="unsupported"></code-example>
+
 ### Write the directive code
 
 Create the directive class file in a terminal window with the CLI command [`ng generate directive`](cli/generate).