Add a comment for path dependencies in plugin example template (#50978)

packages/flutter_tools/templates/app/pubspec.yaml.tmpl

@@ -27,6 +27,11 @@ dependencies:
 
 {{#withPluginHook}}
   {{pluginProjectName}}:
+    # When depending on this package from a real application you should use:
+    #   {{pluginProjectName}}: ^x.y.z
+    # See https://dart.dev/tools/pub/dependencies#version-constraints
+    # The example app is bundled with the plugin so we use a path dependency on
+    # the parent directory to use the current plugin's version. 
     path: ../
 {{/withPluginHook}}