Update a11y design doc?

[pixelzoom]

From #53 (comment):

• @arouinfar Is it worth the time to put this information in the design doc? Alternative Input support #53 (comment)

I don't know how important it is to keep this design document up-to-date, but I think we've made a number of decisions/changes in #53 that are different. Will this be confusing when work on Interactive Descriptions is resumed in the future? Will this same document be used to continue that work?

[arouinfar]

There is a section at the top of the a11y doc for important issues, so I added #53. I also added the step sizes to the sliders and checked that the focus order is correct. I don't believe there's anything else to do for now, so closing.