The Tk commands tk_menuBar and tk_bindForTraversal commands are implemented in the obsolete.tcl file in the Tk package, documented as procs that people should no longer be using, but which are being kept around for backwards compatibility.
Previously the invocation of this Tk command is tk_menuBar. Apparently because these are obsolete, they are no longer being documented.
See also incr Widgets doc. Here's an example of the Iwidgets menubar: http://204.247.240.249/iwidgets/iwidgets/menubar.html
Anyone have a discussion of what a menubar should be?
Tk syntax help - Arts and crafts of Tcl-Tk programming - Category Command, a part of the Tk package