Improve docs around returning data.frames

[bquistorff]

A few small documentation notes might be helpful:

1. The rcall.sthlp still says that R data.frames will be returned as rclass objects, but I think it's only matrices now (from testing and the github readme). This was a bit confusing because it was implied in the Stata article.
2. It might be good to make it clear that st.load doesn't work with Rcall script (I think it's only interactive mode?).