feat(defaults): enable customization of path_display.truncate (#1264)

followup to https://github.com/nvim-telescope/telescope.nvim/pull/1254
and implementation of
https://github.com/nvim-telescope/telescope.nvim/pull/1254#issuecomment-922527270

lua/telescope/config.lua

@@ -257,7 +257,8 @@ append(
   - "shorten"   only display the first character of each directory in
                 the path
   - "truncate"  truncates the start of the path when the whole path will
-                not fit
+                not fit. To increase the the gap between the path and the edge.
+                set truncate to number `truncate = 3`
 
   You can also specify the number of characters of each directory name
   to keep by setting `path_display.shorten = num`.