Well, here's a better way. Still not ideal (where ideal would be
dropping directly onto the .rb itself), but it'll do for now:

D:\<snip>\BindingDocumentation>type MakeDocumentation.bat
pushd %~dp0
ruby MakeDocumentation.rb %1
popd

That will change the working directory to the directory the .bat file
is in, and run the ruby command and pass in the path to the file
dropped on the bat file.