no documentation of datatable.prettyprint.char in vignettes

[tdhock]

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?

[tdhock]

related to #5425

[mahjabinoyshi]

Hi @tdhock! I'd love to work on this task. I plan to add a brief explanation of the option to the datatable-intro vignette as suggested