add docs on relevant browsers in each relevant feature detect

ex. https://github.com/Modernizr/Modernizr/issues/1203 for flexboxtweener, some docs on which browsers this flag would be true or false would be nice.

or maybe this issue is specific to this feature since https://github.com/Modernizr/Modernizr/blob/master/feature-detects/css/flexwrap.js has good docs