no documentation of datatable.prettyprint.char in vignettes

as expected, this important option is prominently mentioned on ?print.data.table ``` Note also the option ‘datatable.prettyprint.char’; character columns entries exceeding this limit will be truncated, with ‘...’ indicating the truncation. ``` however I would suggest adding mention to a vignette. how about in the section of datatable-intro that mentions datatable.print.nrows?