os/exec: document Process.Kill behaviour

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>

src/os/exec.go

@@ -109,7 +109,9 @@ func (p *Process) Release() error {
 	return p.release()
 }
 
-// Kill causes the Process to exit immediately.
+// Kill causes the Process to exit immediately. Kill does not wait until
+// the Process has actually exited. This only kills the Process itself,
+// not any other processes it may have started.
 func (p *Process) Kill() error {
 	return p.kill()
 }