Add information about "Data codoc mismatches" warning

[szimmer]

Add details on handling Data codoc mismatches. Can these be handled by cran-comments or are they required to be fixed?

[Baltmann01]

Good issue, thanks for the suggestion.
Since this is flagged as a WARNING by the CRAN checks and will lead to auto-rejection, it should be fixed and cran-comments won't be enough. In the last 6 months (that's how far my mail history goes back) not one packages was accepted if it had this WARNING and I don't think the CRAN team would accept it.