Docs can be found at

2002-06-21 - Marty Backe's example of displaying a file in a text widget:

 package require Iwidgets
 iwidgets::scrolledtext .t
 pack .t -fill both -expand true

 set filename "readme.txt"
 .t import $filename end

HJG 2016-02-07: It looks like there are problems with Iwidgets.

With tcl 8.6, this example just crashes.
With tcl 8.5, it crashes when the file doesn't exist.

Also, that screenshot at exists, but doesn't show up.