EuiDescribedFormGroup clobbers title semantics

[myasonik]

What's happening?

The title text we render inside of a hidden legend and on the actual contents we add a aria-hidden="true".

On top of limiting now what we can put in titles, having a title also be a heading no longer works which breaks all of Advanced Settings and likely other pages throughout Kibana.

What's the impact?

Large. Not only does this impact all semantically meaningful contents within titles, we've used this pattern in other components as well (though I don't remember which).

In the case of Advanced Settings, previously a user could navigate by heading to any setting. Now that's simply not possible as individual settings don't have a heading.

More ripples from this change, it also makes custom setting inaccessible because they render a tooltip in the heading. Probably not the end of the world of custom settings but it does point to a pattern that I'm sure is in use somewhere else that will be made inaccessible.

[cchaos]

I think that aria-hidden attribute snuck in there from this PR #2783 when the original attempt was to hide the descriptions entirely to use the aria-describedby. @miukimiu Correct me if I'm wrong, but that shouldn't be there, yes?

[cchaos]

Oh wait nm, I walk that back. I see the hidden legend now.

[myasonik]

Added this to the description too we found another impact in Kibana:

More ripples from this change, it also makes custom setting inaccessible because they render a tooltip in the heading. Probably not the end of the world of custom settings but it does point to a pattern that I'm sure is in use somewhere else that will be made inaccessible.