Document "Revoke USB debug authorizations"

It is not clear why the button "Revoke USB debugging authorizations"
may be missing from the Developer settings. This is only adding a
line to explain why we don't display it.

Test: NA
Bug: NA
Change-Id: Ie67e7a8142fc80d35016e0b43febf68d2c0354d1

src/com/android/settings/development/ClearAdbKeysPreferenceController.java

@@ -52,6 +52,9 @@ public class ClearAdbKeysPreferenceController extends DeveloperOptionsPreference
 
     @Override
     public boolean isAvailable() {
+        // If the build is insecure (any -user build, 'ro.adb.secure=0'), adbd does not
+        // requests/store authorizations. There is no need for a "revoke authorizations"
+        // button.
         return AdbProperties.secure().orElse(false);
     }