Update the documentation to clarify about the timer in ISecureClock.

Bug: 197662247
Test: N/A
Change-Id: If35666b6557c432211c75063104c598602f78c87
diff --git a/security/secureclock/aidl/android/hardware/security/secureclock/ISecureClock.aidl b/security/secureclock/aidl/android/hardware/security/secureclock/ISecureClock.aidl
index a742ff0..e6d63c8 100644
--- a/security/secureclock/aidl/android/hardware/security/secureclock/ISecureClock.aidl
+++ b/security/secureclock/aidl/android/hardware/security/secureclock/ISecureClock.aidl
@@ -25,6 +25,10 @@
  * secret. The shared secret must be available to secure clock service by implementing
  * ISharedSecret aidl. Note: ISecureClock depends on the shared secret, without which the secure
  * time stamp token cannot be generated.
+ *
+ * The timer must be the same that is used for HardwareAuthTokens. The ISecureClock interface is
+ * used to convey a fresh timestamp to those components that do not share a timer with the
+ * authenticators.
  * @hide
  */
 @VintfStability