Docs: Mention previous config key for plugins
Docs: Mention previous config key for plugins

fix #6674
x-way/jekyll,chrisfinazzo/jekyll,fulldecent/jekyll,Strangehill/jekyll,getandpost/jekyll.github.io,jaybe-jekyll/jekyll,Strangehill/jekyll,fulldecent/jekyll,tillgrallert/jekyll,tillgrallert/jekyll,jekyll/jekyll,tillgrallert/jekyll,tillgrallert/jekyll,chrisfinazzo/jekyll,getandpost/jekyll.github.io,fulldecent/jekyll,ryanshaw/jekyll,Strangehill/jekyll,Strangehill/jekyll,jaybe-jekyll/jekyll,chrisfinazzo/jekyll,tillgrallert/jekyll,yhironaka/yhironaka.github.io,jaybe-jekyll/jekyll,x-way/jekyll,x-way/jekyll,jekyll/jekyll,fulldecent/jekyll,chrisfinazzo/jekyll,Strangehill/jekyll,getandpost/jekyll.github.io,jekyll/jekyll,x-way/jekyll,ryanshaw/jekyll,jaybe-jekyll/jekyll,ryanshaw/jekyll,jekyll/jekyll,ryanshaw/jekyll