Beefs up variables.md

Adds site.data
Adds site.collections
Adds site.html_pages
Merge branch 'master' of github.com:jekyll/jekyll into beefUpDox
Merge pull request #2567 from austinpray/beefUpDox
Update history to reflect merge of #2567 [ci skip]
Merge branch 'master' of https://github.com/jekyll/jekyll