Document accesskit properties and roles that don't match ARIA

Accesskit types are ported for chromium's API, which don't always match ARIA properties.

I've gone through the entire property list with @DataTriny and we wrote the mismatched properties, though my notes are sparse and I usually didn't note what the exact mismatch was. Still, if we want to document our relationship with ARIA, these properties are a good starting point:

• IsSpellingError
• IsGrammarError
• IsSearchMatch
• IsSuggestion
• Children
• InPageLinkTarget
• MemberOf
• NextOnLine
• PreviousOnLine
• PopupFor
• Label
• Value
• ClassName
• HtmlTag
• Url
• RowIndexText
• ColumnIndexText
• ScrollX
• ScrollXMin
• ScrollXMax
• ScrollY
• ScrollYMin
• ScrollYMax
• NumericValue
• NumericValueStep
• NumericValueJump
• FontSize
• FontWeight
• BackgroundColor
• ForegroundColor
• Overline
• Strikethrough
• Underline
• Invalid
• Toggled
• TextDirection
• ListStyle
• TextAlign
• VerticalOffset
• TextSelection

Ideally we should also document differences in roles and actions, but I haven't started to list those.

Start by reviewing the listed AccessKit properties and comparing each with its corresponding ARIA property or concept. Record the exact mismatches rather than only the names, then determine how roles and actions should be covered. Done means the AccessKit relationship with ARIA is documented for the listed properties, with roles and actions addressed or clearly scoped.