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

doc/telescope.txt

@@ -192,7 +192,8 @@ telescope.setup({opts})                                    *telescope.setup()*
         - "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`.