To anyone interested,
I've just solved this riddle myself by having a very close look at the
source code: Support for two-level nested indices is indeed implemented,
but contrary to intuition (and even contrary to the in-source
documentation) needs the following format:
@addindex command-line
I'm trying to use Doxygen to compile a set of Markdown files into a user
manual in various output formats, including HTML Help. Of couse I want
to make use of HTML Help's index feature.
I have found that I can populate the index using the "@addindex" tag in
places I want indexed. So far so good.
