fixed typos, tried to clarify definition of "local" option

but I am still uncertain what `local == TRUE` does. I was going to add the line `#' Like running \code{R CMD INSTALL <package directory>} without the \code{--build}) option.` but I wasn't sure if this is exactly true. 

It is also not clear what kind of "artifacts" might be left behind (or other undesirable effects).