Update API documentation for onRttInitiationFailure.

Add intdef for reason code.
Clarifying when this callback can be expected and how to handle RTT
state when requested at the start of a call.

Test: Docs only change
Bug: 216608696
Change-Id: I86e46fbb2b0693564a41d1ca0d9f8c180d775c1a
diff --git a/telecomm/java/android/telecom/Connection.java b/telecomm/java/android/telecom/Connection.java
index 467084a..21c6b47 100644
--- a/telecomm/java/android/telecom/Connection.java
+++ b/telecomm/java/android/telecom/Connection.java
@@ -1399,6 +1399,18 @@
          * Session modify request rejected by remote user.
          */
         public static final int SESSION_MODIFY_REQUEST_REJECTED_BY_REMOTE = 5;
+
+
+        /**@hide*/
+        @Retention(RetentionPolicy.SOURCE)
+        @IntDef(prefix = "SESSION_MODIFY_REQUEST_", value = {
+                SESSION_MODIFY_REQUEST_SUCCESS,
+                SESSION_MODIFY_REQUEST_FAIL,
+                SESSION_MODIFY_REQUEST_INVALID,
+                SESSION_MODIFY_REQUEST_TIMED_OUT,
+                SESSION_MODIFY_REQUEST_REJECTED_BY_REMOTE
+        })
+        public @interface RttSessionModifyStatus {}
     }
 
     /**