allow icons for radio field

[matkoniecz]

fixes #290

[k-yle]

i can confirm that there's no JSON schema changes required, so this docs change is sufficient

LGTM since it's documenting an existing practice (although iD doesn't use the icons for radio)

[matkoniecz]

"documenting an existing practice" is overstating it a bit, given it is single PR where I kind of forgot about icons...

but still - icons for radio selectors seem viable, also in iD

[tyrasd]

yep, icons make sense and should also be allowed also for the radio type (as it is essentially just a permanently expanded combo field).