Improve collections docs
Improve collections docs

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

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