DOC: Add %in% operator into compare w r (GH3850)
Doesn't close #3850 but at least the `%in%` operator is now in the comparison with R docs. I've lumped it with the `match` function since thats the page you see the `%in%` operator in the R docs http://finzi.psych.upenn.edu/R/library/base/html/match.html.

@chappers thanks again!
