Various edits to the Intro to RNeXML section 

Includes an additional reference (to TreeBASE).
The edit here isn't quite accurate; I guess the original wording wasn't clear. `get_trees_list()` _always` returns a "list of `multiPhylo` objects. That is, it always has return type of class `list`, whose elements are of class `multiPhylo`, which itself is a kind of list whose elements are of class `phylo`. 

So whether or not there are multiple tree blocks, the command `trees <- get_trees_list()` means that `trees[[1]][[1]]` returns the first tree in the first block, `trees[[1]][[2]]` the second tree in the first block (if it exists), `trees[[2]][[1]]` the first tree in the second block, and so forth.  

@hlapp Wanna take a stab at saying that somewhat more concisely? ;-)