Update documentation to clarify that R.id.* is optimized with aapt2

PiperOrigin-RevId: 257632438
diff --git a/src/main/java/com/google/devtools/build/lib/rules/android/AndroidRuleClasses.java b/src/main/java/com/google/devtools/build/lib/rules/android/AndroidRuleClasses.java
index 8eaff64..9829099 100644
--- a/src/main/java/com/google/devtools/build/lib/rules/android/AndroidRuleClasses.java
+++ b/src/main/java/com/google/devtools/build/lib/rules/android/AndroidRuleClasses.java
@@ -628,7 +628,7 @@
             <li>resources in <code>values/</code> will be removed as well as file based
                 resources</li>
             <li>uses <code>strict mode</code> by default</li>
-            <li>removing unused ID resources is not supported</li>
+            <li>removing unused ID resources is only supported with aapt2</li>
           </ul>
           If resource shrinking is enabled, <code><var>name</var>_files/resource_shrinker.log</code>
           will also be generated, detailing the analysis and deletions performed.