cmd/go: re-run mkalldocs.sh after testflag change

Change-Id: Ia21501df23a91c065d9f2acc6f043019a1419b22
Reviewed-on: https://go-review.googlesource.com/13092Reviewed-by: Andrew Gerrand <adg@golang.org>

src/cmd/go/alldocs.go

@@ -1404,6 +1404,7 @@ control the execution of any test:
 
 	-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.