[Progress bar] clarify `aria-valuenow`, `aria-valuemin`, `aria-valuemax` accessibility info

[laurenmrice]

Acceptance criteria

• Update the A11y tab for Progress Bar in the Development considerations section to indicate you don't have to have the aria-valuenow as well as aria-valuemin, aria-valuemax for indeterminate. Not providing the aria-valuenow is the most important thing you do not need to note here.