add function description to all functions in write_dup_func.R
These functions need a few words description, parameters in and out here:
https://git.noc.ac.uk/brecinosrivas/icoads.utils/-/blob/master/R/write_dup_func.R#L1-7
https://git.noc.ac.uk/brecinosrivas/icoads.utils/-/blob/master/R/write_dup_func.R#L20-26
https://git.noc.ac.uk/brecinosrivas/icoads.utils/-/blob/master/R/write_dup_func.R#L35-41
https://git.noc.ac.uk/brecinosrivas/icoads.utils/-/blob/master/R/write_dup_func.R#L53-58
Needs to be documented also in the Wiki.
Is best if @eck or @ricorne can comment below and I will add it.