face: don't set TransportState to FAILED if connection is closed cleanly

Change-Id: I0989cdf49558574f30b55a172d467b39683f9bb6
Refs: #4099
diff --git a/daemon/face/transport.hpp b/daemon/face/transport.hpp
index ff5363f..85a0fd7 100644
--- a/daemon/face/transport.hpp
+++ b/daemon/face/transport.hpp
@@ -40,9 +40,9 @@
  */
 enum class TransportState {
   NONE,
-  UP, ///< the transport is up
-  DOWN, ///< the transport is down temporarily, and is being recovered
-  CLOSING, ///< the transport is requested to be closed
+  UP, ///< the transport is up and can transmit packets
+  DOWN, ///< the transport is temporarily down, and is being recovered
+  CLOSING, ///< the transport is being closed gracefully, either by the peer or by a call to close()
   FAILED, ///< the transport is being closed due to a failure
   CLOSED ///< the transport is closed, and can be safely deallocated
 };