Update docstring of graphviz_draw.

The effect of setting "output" to None should be more clear now.
4 jobs for patch-2 in 89 minutes and 35 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #5034
amd64
job_clang_amd64

00:34:13

passed #5029
amd64
job_clang_cxx_amd64

00:32:36

passed #5027
amd64
job_gcc_amd64

00:55:21

failed #5028
amd64
job_clang_amd64

01:05:00