cmd/go: documented default value of the -timeout testflag

Change-Id: I4dc75065038a9cfd06f61c0deca1c86c70713d3a
Reviewed-on: https://go-review.googlesource.com/13091Reviewed-by: Andrew Gerrand <adg@golang.org>

src/cmd/go/test.go

@@ -220,6 +220,7 @@ const testFlag2 = `
 
 	-timeout t
 	    If a test runs longer than t, panic.
+	    The default is 10 minutes (10m).
 
 	-trace trace.out
 	    Write an execution trace to the specified file before exiting.