Add aliases in roxygen docs for R H2OFrame functions
There are a bunch of H2OFrame functions for which we have two aliases, e.g. `h2o.abs()` and `abs()`. However, if you look in the R docs for `abs()` it only sends you to the R data.frame version. We should add an alias so that when you type, `?abs()` it will show you both versions (base R and H2O) so you can get to the h2o.abs() docs more easily.