Improve uncontrolled form components documentation

Referring to Uncontrolled Components I think there are some pitfalls to be documented when using them.

Rendering uncontrolled components correctly

In contrast to controlled components, uncontrolled components using defaultValue, defaultChecked should not be rendered until the default is present, respectively <select> should not be rendered until the default and the <option> children are present, because the functionality of the defaultValue or defaultChecked is only given after mounting, not on updating.

Not being rendered can be achieved in two different ways:

1. simply don't render it e.g. return null
2. force rerender with changing key on it

If you strictly don't want partial results in your form use "B", else "C" would work.

// A)
// bad
class UncontrolledMotd extends React.Component {

  state = {
    text: null
  };

  async componentDidMount() {
    const res = await fetch('/motd');
    const text = await res.text();
    this.setState({text});
  }

  render() {
    // bad default
    return (
      <input type="text" defaultValue={this.state.text} />
    );
  }
}

// B)
// better: no render, no placeholder
class UncontrolledMotd extends React.Component {

  state = {
    loaded: false,
    text: null
  };

  async componentDidMount() {
    const res = await fetch('/motd');
    const text = await res.text();
    this.setState({loaded: true, text});
  }

  render() {
    if (!this.state.loaded) {
      return null;
    }

    return (
      <input type="text" defaultValue={this.state.text} />
    );
  }
}

// C)
// better: placeholder, using key
class UncontrolledMotd extends React.Component {

  state = {
    loaded: false,
    text: null
  };

  async componentDidMount() {
    const res = await fetch('/motd');
    const text = await res.text();
    this.setState({loaded: true, text});
  }

  render() {
    return (
      <input key={!this.state.loaded ? 'preview' : 'final'} type="text" defaultValue={this.state.text} />
    );
  }
}

Did not test code above, but the idea in my current project and it works.
What do you think?

Regards Philipp

Comienza revisando la documentación enlazada sobre Uncontrolled Components y los ejemplos propuestos en este issue. Verifica el comportamiento de los componentes no controlados y determina cómo deberían encajar los posibles problemas en la documentación; se considera terminado cuando las indicaciones relevantes y los ejemplos probados se hayan incorporado claramente.