[R] unexpected conversion from POSIXlt to POSIXct

[Paul MacManus]

The documentation for DateTime classes in R 2.3.0 says that limited
arithmetic is available for both POSIXlt and POSIXct. It needs to make
clear that POSIXlt can be converted to POSIXct without warning under
arithmetic operations

EXAMPLE

ISOdatetime(2003, 1, 1, 0, 0,0) -> dd
as.POSIXlt(dd) -> dd          ## set class of dd to POSIXlt
dd+1 -> ee                        ## ee now has class POSIXct