Generate docstring to use in the ds_ files

Generate docstring to use in the ds_ files

ds_docstring(ds, name, desc, url, ...)

Arguments

ds	the name of the ds_ R function
name	the name of the data provider
desc	the name(s) of the countries supported by the data provider, e.g., "United States".
url	the link to the data provider
...	arguments passed to the ds_ function