a11y-doc: Adjust Focus Control to new 16.3 refs + link fixes.

[AlmeroSteyn]

With release of React 16.3 and the changes to the ref API I have adjusted the accessibility doc to reflect this.

I also fixed a few dead/broken links I found on this page.

I have chosen not to refer to forwardRef in the a11y doc as it is more for creators of HOC's and trying to explain it again on this page would detract from achieving accessibility IMHO.

Content changes are limited to the Programmatically managing focus section. Changes are summarised in the following captures:

[reactjs-bot]

Deploy preview for reactjs ready!

Built with commit 6883e33

https://deploy-preview-742--reactjs.netlify.com

[AlmeroSteyn]

NOTE: I accidentally duplicated #708 in this fix.

[gaearon]

Maybe we should promote React.forwardRef() here instead?

[AlmeroSteyn]

@gaearon I have adjusted the part about HOC's to mention ref forwarding.

To avoid too much code example duplication and keep focus on the a11y issue at hand I have still not added a code example as this can be found through the link.

The sentence has now become:

When using a HOC to extend components, it is recommended to forward the ref to the wrapped component using the forwardRef function of React. If a third party HOC
does not implement ref forwarding, the above pattern can still be used as a fallback.

What do you think?

[gaearon]

Sounds good. Mind rebasing?

[AlmeroSteyn]

Done!

[AlmeroSteyn]

Wow that was super quick! Thank you!