docs: Added perm req for getUserCount()

Test: make ds-docs -j32

Bug: 31091793

Change-Id: Ib1cae91795b0c054f2ae95af6376a8f01d839d9f
diff --git a/core/java/android/os/UserManager.java b/core/java/android/os/UserManager.java
index 9c9829f..96f5ec4 100644
--- a/core/java/android/os/UserManager.java
+++ b/core/java/android/os/UserManager.java
@@ -2337,6 +2337,8 @@
 
     /**
      * Return the number of users currently created on the device.
+     * <p>This API is not for use by third-party apps. It requires the {@code MANAGE_USERS}
+     * permission.</p>
      */
     public int getUserCount() {
         List<UserInfo> users = getUsers();