os: document that Process.Wait only works on child processes

Fixes #7072

LGTM=iant
R=iant
CC=golang-codereviews
https://golang.org/cl/71900045

src/pkg/os/doc.go

@@ -39,6 +39,8 @@ func (p *Process) Kill() error {
 // Wait waits for the Process to exit, and then returns a
 // ProcessState describing its status and an error, if any.
 // Wait releases any resources associated with the Process.
+// On most operating systems, the Process must be a child
+// of the current process or an error will be returned.
 func (p *Process) Wait() (*ProcessState, error) {
 	return p.wait()
 }