I noticed - and came across the same problem - people on comp.lang.tcl are regularly asking for tools to automatically generate documentation from source code. It seems no page is currently dedicated to maintain a list of tools available for Tcl. So let's start here (in no particular order):
escargo 21 Apr 2005 - One thing to keep in mind with these tools is that some of them might be used with OO systems that extend the Tcl syntax, Snit for example. Now, instead of just need to track proc definitions, namespaces, and global variables, you might want to keep track of classes, methods, typemethods, and other interesting extended Tcl syntax. I haven't looked at all of these tools, but I know that robodoc made it possible to handle pretty arbitrary things to track and document.
MAKR, 2006-12-08: Half a year ago I decided to move over to robodoc. I now have all in-source documentation (C and Tcl) converted to this format. This was one of the best ideas I ever had. The commentary in the sources is clean and readable. The resulting HTML documentation is it too. You can format it to your liking using CSS...
Tcl Style Guide has a couple of recommendations about documentation near the end of the page.
There is, on the wiki, a page about docbook, however, I'm uncertain how much use of docbook occurs in the tcl development community efforts to document tcl.
GSoC 2011 has a project proposal in this direction: GSoC Idea: Auto Documentation Tool for Tcl