docs(@angular/cli): add doc for --delete-output-path option

Author: jhnoor

docs/documentation/build.md

@@ -223,6 +223,16 @@ Note: service worker support is experimental and subject to change.
   </p>
 </details>
 
+<details>
+  <summary>delete-output-path</summary>
+  <p>
+    <code>--delete-output-path</code> (aliases: <code>-dop</code>) <em>default value: true</<em>
+  </p>
+  <p>
+    Delete the output-path directory.
+  </p>
+</details>
+
 <details>
   <summary>poll</summary>
   <p>