supported-formats.txt: clarify ratings legend

It is now called "Ratings legend and definitions" so that people know
that clicking the link will also explain the column headers, too.

Thanks to Helen Flynn for the suggestion.
supported-formats.txt: add note about sorting

This adds a note (to the HTML version only) that you can sort by any of
the columns by clicking on its header.

Thanks to Helen Flynn for the suggestion.
Add sorting note to other sortable tables, too