Added role='dialog' to EuiFlyout and documented correct way to make it announce itself with aria attributes

[cjcenizal]

Fixes #725

[cjcenizal]

jenkins, test this

[cchaos]

It now reads the description twice:

https://d.pr/free/v/3smVQ5

[cjcenizal]

Thanks @cchaos. Looks like aria-describedby was the culprit so I removed it since it seems to be optional. Can you take another look?

[cchaos]

So the only thing I see now is that when I click the show button it starts reading all the contents of the flyout. Then if I tab to any tabbable contents within the flyout, it will start re-reading all the contents of the flyout.

[cjcenizal]

I'm not 100% sure but I think that might be "normal" behavior. The Bootstrap modal has the same issue, where you click on the button and the entire modal content gets read, and then you tab into the modal and the whole thing gets read again. Tabbing through the buttons inside of the modal is fine though, and that's the same with the flyout. So I think this is OK.

[cchaos]

Yeah it looks like in the modal patterns here: https://a11yproject.com/patterns#modal-windows it does the same thing. This looks good for this, though we may want to consider writing up some more docs on using aria-labelledby and aria-describedby specifically for flyouts.