Rdoc-swig is a project mainly written in Ruby, it's free.
RDoc (Ruby 1.8) extension to parse documentation directly from SWIG files
Generating RDoc (Ruby 1.8) documentation from SWIG input (*.i) files
Since I made the experience that using SWIGs %doc mechanism is clumsy to use, I did a closer look at the the parsing files of RDoc.
rdoc_swig is the result. It directly parses comments from a SWIG input (typically .i) file and generates RDoc documentation.
Just call the local 'rdoc' script. It will pick up the parse_swig.rb file and call the installed 'rdoc'. There's no difference to behavior of the normal 'rdoc' tool.