os/exec: explicitly mention Cmd.Wait() has to be called eventually

LGTM=minux.ma, r
R=golang-codereviews, minux.ma, r
CC=golang-codereviews
https://golang.org/cl/67280043

src/pkg/os/exec/exec.go

@@ -238,6 +238,9 @@ func (c *Cmd) Run() error {
 }
 
 // Start starts the specified command but does not wait for it to complete.
+//
+// The Wait method will return the exit code and release associated resources
+// once the command exits.
 func (c *Cmd) Start() error {
 	if c.lookPathErr != nil {
 		c.closeDescriptors(c.closeAfterStart)
@@ -304,6 +307,8 @@ func (e *ExitError) Error() string {
 // If the command fails to run or doesn't complete successfully, the
 // error is of type *ExitError. Other error types may be
 // returned for I/O problems.
+//
+// Wait releases any resources associated with the Cmd.
 func (c *Cmd) Wait() error {
 	if c.Process == nil {
 		return errors.New("exec: not started")