docs: clarify usage of body overflow in XY Grid grid-frame example
docs: clarify usage of body overflow in XY Grid grid-frame example

See [comment](https://github.com/zurb/foundation-sites/issues/11191#issuecomment-389658664).

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