Fixed typo for reduceSeries
Improved documentation for areaBetween
Improved documentation for cactiStyle

Explained possible options for system (not letting the reader hunt around for
possible values), as well as culled most info for alignment from:
https://github.com/graphite-project/graphite-web/issues/671
Merge pull request #839 from ojilles/master

Documentation updates for areaBetween, cactiStyle and reduceSeries