clarify _config.yml/collections type (#7873)
clarify _config.yml/collections type (#7873)

Merge pull request 7873
fulldecent/jekyll,jekyll/jekyll,tillgrallert/jekyll,jaybe-jekyll/jekyll,ryanshaw/jekyll,fulldecent/jekyll,jaybe-jekyll/jekyll,tillgrallert/jekyll,ryanshaw/jekyll,tillgrallert/jekyll,x-way/jekyll,yhironaka/yhironaka.github.io,jekyll/jekyll,fulldecent/jekyll,jekyll/jekyll,x-way/jekyll,fulldecent/jekyll,jaybe-jekyll/jekyll,ryanshaw/jekyll,tillgrallert/jekyll,chrisfinazzo/jekyll,x-way/jekyll,chrisfinazzo/jekyll,jaybe-jekyll/jekyll,ryanshaw/jekyll,chrisfinazzo/jekyll,x-way/jekyll,tillgrallert/jekyll,chrisfinazzo/jekyll,jekyll/jekyll