setting the page size and margins would be a basic feature.
Gavin already explained the paper story, but let me add that the purpose
of texinfo[.tex] is not to typeset any document whatsoever, like (La)TeX
or Lout or <whatever>, but to generate software documentation in
multiple output formats. Therefore there are quite a number of general
formatting features not present. E.g., no way to switch fonts.
It would be theoretically possible to set all the page layout parameters
by calling \internalpagesizes inside @tex, but since that is, well,
internal, the interface requires understanding TeX's page primitives, so
it doesn't really seem appropriate for your case. (See the definition of
\internalpagesizes in texinfo.tex if you want to pursue it anyway,)
I wonder if the OpenOCD project is doing this the wrong way. Is there
perhaps a better way to generate a PDF from a Texinfo file?
Maybe @afourpaper will give better results for your document?
Alternatively, for general control over the typeset results, generating
LaTeX with texi2any and including desired packages is more general. For
example, \usepackage{xurl} will give you the behavior you suggested of
breaking urls at any character. It would not make sense, or be feasible,
to reimplement every document feature in the world in Texinfo.
Or is Texinfo not really suitable for PDF documentation?
Virtually every manual I've seen in Texinfo generates PDF, e.g.,
subdirectories of gnu.org/software, latexref.xyz, etc. For the documents
I myself have worked on, yes, I do have to rewrite text sometimes to
make breaks of long urls or identifiers come out reasonably -- not just
for Texinfo, but also (La)TeX. So it goes. --happy formatting, karl.
