Revise MSB3189 documentation date and add description

Updated the date for the MSB3189 error documentation and added a description of the error's cause.

Author: ghogen

docs/msbuild/errors/msb3189.md

@@ -1,7 +1,7 @@
 ---
 title: "MSB3189 diagnostic code"
 description: Learn about the possible causes of the MSB3189 build error, and get troubleshooting tips.
-ms.date: 05/16/2025
+ms.date: 9/29/2025
 ms.topic: "error-reference"
 f1_keywords:
  - MSB3189
@@ -32,6 +32,9 @@ This article describes the MSB3189 error code.
 <!-- MSB3189: The update location for this application is a local path. -->
 
 <!-- :::editable-content name="postOutputDescription"::: -->
+## Description
+
+This error is given when the ClickOnce deployment path is not in one of the supported formats. The string must be a fully qualified URL or UNC path, for example "http://www.contoso.com/myapplication" or "\\server\myapplication". Local paths and mapped network drives are not supported.
 <!--
 {StrBegin="MSB3189: "}
 -->