Improve theme docs
Improve theme docs

See https://github.com/jekyll/jekyll/pull/5630 for more details on the update. 

@jekyll/documentation
@DirtyF
fulldecent/jekyll,xiebinhqy/jklly_china,Strangehill/jekyll,jekyll/jekyll,jekyll/jekyll,rasa2011/rasa2011.github.io,tillgrallert/jekyll,gencer/jekyll,tillgrallert/jekyll,rasa2011/rasa2011.github.io,alex-kovac/jekyll,jaybe-jekyll/jekyll,jaybe-jekyll/jekyll,x-way/jekyll,tomjohnson1492/jekyll,tillgrallert/jekyll,ryanshaw/jekyll,jaybe-jekyll/jekyll,jekyll/jekyll,getandpost/jekyll.github.io,xiebinhqy/jklly_china,chrisfinazzo/jekyll,rlugojr/jekyll,getandpost/jekyll.github.io,jekyll/jekyll,rasa2011/rasa2011.github.io,ryanshaw/jekyll,xiebinhqy/jklly_china,gencer/jekyll,alex-kovac/jekyll,tillgrallert/jekyll,x-way/jekyll,tillgrallert/jekyll,tomjohnson1492/jekyll,tomjohnson1492/jekyll,chrisfinazzo/jekyll,fulldecent/jekyll,Strangehill/jekyll,alex-kovac/jekyll,x-way/jekyll,rlugojr/jekyll,gencer/jekyll,chrisfinazzo/jekyll,jaybe-jekyll/jekyll,alex-kovac/jekyll,xiebinhqy/jklly_china,Strangehill/jekyll,fulldecent/jekyll,gencer/jekyll,x-way/jekyll,Strangehill/jekyll,Strangehill/jekyll,fulldecent/jekyll,alex-kovac/jekyll,ryanshaw/jekyll,yhironaka/yhironaka.github.io,getandpost/jekyll.github.io,gencer/jekyll,ryanshaw/jekyll,rlugojr/jekyll,chrisfinazzo/jekyll,tomjohnson1492/jekyll,tomjohnson1492/jekyll,rlugojr/jekyll