trace: clarify how gRPC options work

Change-Id: Iafca1d6643a8a6b81b6b7b1035e9ef7f4413768f
Reviewed-on: https://code-review.googlesource.com/11310
Reviewed-by: kokoro <[email protected]>
Reviewed-by: Hana Kim <[email protected]>

Author: rakyll

trace/trace.go

@@ -230,13 +230,16 @@ func requestHook(ctx context.Context, req *http.Request) func(resp *http.Respons
 	}
 }
 
-// EnableGRPCTracingDialOption enables tracing of requests that are sent over a
-// gRPC connection.
+// EnableGRPCTracingDialOption traces all outgoing requests from a gRPC client.
+// The calling context should already have a *trace.Span; a child span will be
+// created for the outgoing gRPC call. If the calling context doesn't have a span,
+// the call will not be traced.
+//
 // The functionality in gRPC that this relies on is currently experimental.
 var EnableGRPCTracingDialOption grpc.DialOption = grpc.WithUnaryInterceptor(grpc.UnaryClientInterceptor(grpcUnaryInterceptor))
 
-// EnableGRPCTracing enables tracing of requests for clients that use gRPC
-// connections.
+// EnableGRPCTracing automatically traces all gRPC calls from cloud.google.com/go clients.
+//
 // The functionality in gRPC that this relies on is currently experimental.
 var EnableGRPCTracing option.ClientOption = option.WithGRPCDialOption(EnableGRPCTracingDialOption)