By digging through some Question and Answers from here, I stumbled upon the MWE in this question. The part
% \begin{function}{\mymacro}
% \begin{syntax}
% \cs{mymacro}\marg{Arg1}
% \end{syntax}
% Descriptive text ...
% ...
% \end{function}
was new to me. I searched intensively the Net, to find some documentation about this cool new stuff. I read expl3.pdf, source3.pdf, l3styleguide.pdf as well as docstrip.pdf and doc.pdf from my 2021 TeXLive system, ... but didn't find an in detail documentation about this new features.
I even digged into source3body.tex. There I got some more hints about how to use the \begin{function}[<options>] ... \end{function} environment. I was surprised to find also a \begin{variable} ... \end{variable} environment.
But I haven't found a basic LaTeX document on how to use these new functions. File l3styleguide.pdf is the one, which comes next to the desired doc, but it still does not document the above listed environments in detail.
Is there a source, I haven't found? Or is this detailed documentation still missing?
l3doc.pdfand notl3pdf.doc? :) – Jan Jan 27 '22 at 23:15l3doc.dtxin a recent TeXLive in$TEXMFDIST/source/latex/l3kernel/l3doc.dtx. I copied to an writeable directory and run it through latex and voila, I got some more interesting stuff to read. Thank you Joseph. – Jan Jan 27 '22 at 23:25