Misleading future prop checking entry in docs

[Marc-Andre-Rivet]

In http://dash.plotly.com/react-for-python-developers, there is an entry for:

In the future, we will use these propTypes to provide validation in Python. That is, if you specify that a property is a PropTypes.string, then Dash's Python code will throw an error if you supply something else. Track our progress in this issue: 264.

This is not accurate as it is already implemented and rather than being exposed in Python it handled in JS and made available through the DevTools.

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Let's make this wider reaching and check up on any future or Dash v x.x references and make sure they still make sense. Especially references to Dash v0.x exception or feature availability.