docs: add docs about "mobile scroll" bug #9707 in OffCanvas
docs: add docs about "mobile scroll" bug #9707 in OffCanvas

Add documentation for the OffCanvas private methods `_recordScrollable` and `_stopScrollPropagation` to explicitely justify the need for `stopPropagation`.

See #9707

abdullahsalem/foundation-sites,zurb/foundation,denisahac/foundation-sites,zurb/foundation-sites,denisahac/foundation-sites,DaSchTour/foundation-sites,abdullahsalem/foundation-sites,DaSchTour/foundation-sites,zurb/foundation,denisahac/foundation-sites,colin-marshall/foundation-sites,colin-marshall/foundation-sites,zurb/foundation-sites,zurb/foundation-sites,zurb/foundation,colin-marshall/foundation-sites,abdullahsalem/foundation-sites