Conflicting docs on sizeMapping

[hayleybech]

We had trouble getting the sizeMapping to work when following your sizeMapping, until we clicked the links to read Google's own documentation. That's when we realised it seems to work by specifying minimum viewport sizes not maximum sizes.

Your docs and examples on this page https://react-dfp.surge.sh/adslot/sizemapping/ show viewports working as maximum widths, however Google's docs and examples here https://developers.google.com/publisher-tag/guides/ad-sizes#responsive_ads show them working as minimum widths, and after testing, that does seem to be the case.