docs: add datatable.prettyprint.char to datatable-intro vignetteFix/prettyprint char vignette docs

vignettes/datatable-intro.Rmd

@@ -95,6 +95,18 @@ You can also convert existing objects to a `data.table` using `setDT()` (for `da
     getOption("datatable.print.nrows")
     ```
 
+* The option `datatable.prettyprint.char` controls how many characters
+  are displayed per entry in character columns when printing. Entries
+  exceeding this limit are truncated with `...` (via `strtrim()`).
+  You can set it like so:
+
+````{r}
+    old = options(datatable.prettyprint.char = 5L)
+    DT = data.table(x = 1:2, y = c("abcdefghij", "klmnopqrstuv"))
+    DT
+    options(old)  # reset to default
+````
+
 * `data.table` doesn't set or use *row names*, ever. We will see why in the [`vignette("datatable-keys-fast-subset", package="data.table")`](datatable-keys-fast-subset.html) vignette.
 
 ### b) General form - in what way is a `data.table` *enhanced*? {#enhanced-1b}