options(datatable.prettyprint.char) is not documented

[MichaelChirico]

          Thanks for the investigation! I confirm your findings and agree that it may not be so simple as just changing to `type='width'`. `substr()` is also involved; note this from `?substr`:

These functions are often used with nchar to truncate a display. That does not really work (you want to limit the width, not the number of characters, so it would be better to use strtrim), but at least make sure you use the default nchar(type = "c").

PS, it seems options(datatable.prettyprint.char) is completely undocumented, so we should address that too.

Originally posted by @MichaelChirico in #5096 (comment)