zulip_plugin [nfc]: Fix comment that said the library has no code

It has ZulipShimPlugin, which forwards to the app's ZulipPlugin, and
the settings in this file are used to compile it. Point to the
package's pubspec.yaml for why the shim exists.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: chrisbobbe

packages/zulip_plugin/android/build.gradle.kts

@@ -5,9 +5,8 @@ plugins {
 android {
     namespace = "com.zulip.flutter"
 
-    // This Gradle project is an empty placeholder that contains no actual code,
-    // so the settings below have no real effect.  The Gradle project exists only
-    // because the Flutter Gradle plugin expects every Flutter plugin to have one.
+    // This Gradle project holds only ZulipShimPlugin, which forwards to the
+    // app's ZulipPlugin. For why, see this package's pubspec.yaml.
 
     compileSdk = flutter.compileSdkVersion