os/exec: document that Cmd.Wait waits for stdin I/O

Fixes #10338.

Change-Id: Ib86cb9a6c694b1e442a9957153c7ca38a7d11c3e
Reviewed-on: https://go-review.googlesource.com/12232Reviewed-by: Andrew Gerrand <adg@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/os/exec/exec.go

@@ -363,6 +363,10 @@ func (e *ExitError) Error() string {
 // error is of type *ExitError. Other error types may be
 // returned for I/O problems.
 //
+// If c.Stdin is not an *os.File, Wait also waits for the I/O loop
+// copying from c.Stdin into the process's standard input
+// to complete.
+//
 // Wait releases any resources associated with the Cmd.
 func (c *Cmd) Wait() error {
 	if c.Process == nil {