Modal - Add content for elementToFocus prop in a11y docs

[thatblindgeye]

Is this a new page or a change to an existing one?
Modal a11y docs

Describe the enhancement
We should add content in the a11y docs related to the elementToFocus prop introduced in React, which allows customizing what receives focus when the Modal is opened + what should typically receive focus.

Are there visuals for this enhancement? If applicable, please include examples for each state and for varying widths
Include screenshots or links to Marvel or other mockups.

Any other information?