docs(@angular-devkit/build-angular): add more info link to proxyConfig

Closes #20827

(cherry picked from commit 7587807)

Author: alan-agius4

packages/angular_devkit/build_angular/src/dev-server/schema.json

@@ -21,7 +21,7 @@
     },
     "proxyConfig": {
       "type": "string",
-      "description": "Proxy configuration file."
+      "description": "Proxy configuration file. For more information, see https://angular.io/guide/build#proxying-to-a-backend-server."
     },
     "ssl": {
       "type": "boolean",