docs: Mention app query filters on Android 11

For apps that target Android 11 and higher, the methods in this class
each return a filtered list by default, because of the new package
visibility behavior.

Test: m ds-docs-java

Bug: 173104139

Exempt-From-Owner-Approval: Docs-only change
Change-Id: Idd239a6a9b4e1764b8285f73a341adc024281be2

core/java/android/content/pm/PackageManager.java

@@ -92,6 +92,11 @@ import java.util.Set;
  * packages that are currently installed on the device.
  *
  * You can find this class through {@link Context#getPackageManager}.
+ *
+ * <p class="note"><strong>Note: </strong>If your app targets Android 11 (API level 30) or
+ * higher, the methods in this class each return a filtered list of apps. Learn more about how to
+ * <a href="/training/basics/intents/package-visibility">manage package visibility</a>.
+ * </p>
  */
 public abstract class PackageManager {
     private static final String TAG = "PackageManager";