doc: streams must be open to be passed to child

spawn stdio options can be a 'stream', but the following code
fails with "Incorrect value for stdio stream: [object Object]",
despite being a stream. The problem is the test isn't really
for a stream, its for an object with a numeric `.fd` property,
and streams do not have an fd until their async 'open' event
has occurred. This is reasonable, but was not documented.

    child_process.spawn('date', [], {stdio: [
      'ignore',
      fs.createWriteStream('out.txt',{flags:'a'}),
      'ignore']})