Document new tabIndex property for radio component

[blackbaud-johnly]

blackbaud/skyux-forms#2

The tabIndex input for the radio component has been deprecated in favor of using the new tabIndex input on the radio group component. The tabIndex represents the position an element is within tab navigation on the page. (tab navigation is when you press tab or shift+tab and focus shifts between elements on the page). A tabIndex of -1 removed the element from tab navigation entirely.

According to accessibility guidelines, focus on a group of radio elements should only ever focus the currently selected radio button or if one isn't selected, the first/last button depending on the direction navigated from. So the tabIndex of a group of radio buttons has been moved to the radio-group element, since individual tabIndexes do not make sense.

[blackbaud-johnly]

Addressed in blackbaud/skyux2-docs#200.