Clarifying the R.chain documentation and example

The documentation for chain is a little confusing as is. The description implies that the input function needs to map a nested list to a nested list. This is one valid use case, but definitely not the only one.

I changed the description of the function to Haskell's documentation for [concatMap](http://hackage.haskell.org/package/base-4.7.0.1/docs/Prelude.html#v:concatMap) and changed the example to include a function that works on a flat input list.
that's nice! i don't see any obvious reason not to do it that way