trusty/apploader: Add missing doc for enum value

Document the APPLOADER_ERR_POLICY_VIOLATION value in enum
apploader_error.

Bug: 208968719
Change-Id: Ia9b17f4ea705d13567b2ba74f2dcd6df5a0c7d73
(cherry picked from commit c5253819f85486ee40ae13f59742afeba8f8ef58)
Merged-In: Ia9b17f4ea705d13567b2ba74f2dcd6df5a0c7d73
diff --git a/trusty/apploader/apploader_ipc.h b/trusty/apploader/apploader_ipc.h
index 306596e..ed5cbde 100644
--- a/trusty/apploader/apploader_ipc.h
+++ b/trusty/apploader/apploader_ipc.h
@@ -45,6 +45,9 @@
  * @APPLOADER_ERR_INTERNAL:             miscellaneous or internal apploader
  *                                      error not covered by the above
  * @APPLOADER_ERR_INVALID_VERSION:      invalid application version
+ * @APPLOADER_ERR_POLICY_VIOLATION:     signature verification succeeded but
+ *                                      key+manifest combination not allowed
+ *                                      by app loader policy engine
  */
 enum apploader_error : uint32_t {
     APPLOADER_NO_ERROR = 0,