doc: script with spaces spawn example for windows

Adds an example of how to spawn a shell script under Windows with
spaces in its filename.

Ref: https://github.com/nodejs/node/issues/7367
PR-URL: https://github.com/nodejs/node/pull/8035
Reviewed-By: João Reis <reis@janeasystems.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/child_process.md

@@ -82,7 +82,8 @@ be launched using [`child_process.execFile()`][]. When running on Windows, `.bat
 and `.cmd` files can be invoked using [`child_process.spawn()`][] with the `shell`
 option set, with [`child_process.exec()`][], or by spawning `cmd.exe` and passing
 the `.bat` or `.cmd` file as an argument (which is what the `shell` option and
-[`child_process.exec()`][] do).
+[`child_process.exec()`][] do). In any case, if the script filename contains
+spaces it needs to be quoted.
 
 ```js
 // On Windows Only ...
@@ -110,6 +111,13 @@ exec('my.bat', (err, stdout, stderr) => {
   }
   console.log(stdout);
 });
+
+// Script with spaces in the filename:
+const bat = spawn('"my script.cmd"', ['a', 'b'], { shell:true });
+// or:
+exec('"my script.cmd" a b', (err, stdout, stderr) => {
+  // ...
+});
 ```
 
 ### child_process.exec(command[, options][, callback])