The spanish indexer is only downloadable for members of the Embarcadero (former Borland) community, it was an undocumented tool with minimal sources and executable. It is unable to descend into subdirectories because it uses a file list box (reflecting files in a directory directly) as selection of sources to be processed.
http://cc.embarcadero.com/Item/18035
I read in overview.htm that it was autogenerated by wilbertd; so if the documentation is automatically generated, this document generator will probably know best which structure the TOC should have, and which relevant keywords exist. But if I had to write a new tool which scans the available documentation, I would probably make it scan overview.htm and quick_ref.htm (at least) with specific previous knowledge.