It is not clear from documentation what the Process.Kill does. And it
leads to reccuring confusion about Cmd.Start/Wait methods.

Fixes #24220

Change-Id: I66609d21d2954e195d13648014681530eed8ea6c
Reviewed-on: https://go-review.googlesource.com/98715Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>