Tweak G13, on-input understanding, F37

[patrickhlauke]

• in G13, as radio buttons/inputs would cause 2.1.1 Keyboard failures (as keyboard users are not able to explicitly select a radio button with the keyboard without triggering a change event, which would then lead to a form submission or page reload in these examples), changing the examples to a select and a set of toggle buttons
• reformatting/cleanup of the markup for G13
• in on-input understanding, replacing the mention of radio buttons with a select dropdown as well, for clarity/consistency (in this case, even the radio button example would be fine, but it may confuse matters)
• in F37, add an extra note explicitly mentioning that radio buttons will still fail 2.1.1 even if they included a warning that satisfies 3.2.2

Closes #898

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[patrickhlauke]

Do we think we need some extra call-out/note in the actual 3.2.2 understanding, warning that a set of radio buttons that have an onchange handler can still end up causing a 2.1.1 Keyboard failure? or is that a bit too specific?

Alternatively, might be worth also revisiting https://www.w3.org/WAI/WCAG21/Techniques/failures/F37 which does seem to suggest that if a set of radio buttons has a warning, it would then not fail ... maybe squeezing in a note at the end there saying that while that will then pass On Input, it will then fail Keyboard even with that preceding warning

EDIT: done