daemon: fix docstring for latencies in protobuf file (scionproto#4443)

Update the docstring to follow the semantics of negative means undefined.

Author: mlegner

proto/daemon/v1/daemon.proto

@@ -74,7 +74,8 @@ message Path {
     // Latency lists the latencies between any two consecutive interfaces.
     // Entry i describes the latency between interface i and i+1.
     // Consequently, there are N-1 entries for N interfaces.
-    // A 0-value indicates that the AS did not announce a latency for this hop.
+    // A negative value indicates that the AS did not announce a latency for
+    // this hop.
     repeated google.protobuf.Duration latency = 6;
     // Bandwidth lists the bandwidth between any two consecutive interfaces, in
     // Kbit/s.