nettest: update comment on MakePipe

To warn about the provided stop function should not be nil.

Change-Id: I73351b6379ae22e85298ce9e87ee80077f748c7e
Reviewed-on: https://go-review.googlesource.com/34091
TryBot-Result: Gobot Gobot <gobot@golang.org>
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
1 file changed