os/exec: clarify Cmd.Wait documentation a bit more explicitly

It already implied that Cmd.Wait is more than os.Process.Wait, but say
so explicitly.

See #18874 (comment)

Updates #18874

Change-Id: Iaa46defd776ae0be817d9f4466a99ac78cfd672b
Reviewed-on: https://go-review.googlesource.com/47650
Reviewed-by: Russ Cox <rsc@golang.org>

Author: bradfitz

Diff for: src/os/exec/exec.go

@@ -416,8 +416,10 @@ func (e *ExitError) Error() string {
 	return e.ProcessState.String()
 }
 
-// Wait waits for the command to exit.
-// It must have been started by Start.
+// Wait waits for the command to exit and waits for any copying to
+// stdin or copying from stdout or stderr to complete.
+//
+// The command must have been started by Start.
 //
 // The returned error is nil if the command runs, has no problems
 // copying stdin, stdout, and stderr, and exits with a zero exit