dillon1000/react

Update example snippet in old 'React.addons' doc page (#9363)

* Update example snippet in old 'React.addons' doc page

This makes the example more consistent.

* Add back the pointers in docs that were mistakenly removed

In https://github.com/facebook/react/pull/9359 we accidentally removed
pointers in some doc pages. Putting them back now.

* Link to npm package instead of github page

This seems like a more stable place to link to in the 'context'
document.
Based on @bvaughn's feedback in https://github.com/facebook/react/pull/9359