How would this feature be useful?
nox -l is great; one of the best features is the fact that it shows the help text from the session's docstrings. It makes it a great entrypoint (and if there was a way to make the "default" be to show the sessions, such as nox.sessions = "-l", I'd probably tend to use that in most of my noxfiles). However, it would be very nice if there was a way to add general information at the top or bottom (don't care which) of that printout. For example, I often would like to add a little help snippet like this:

Run on all versions of Python with:
  docker run --rm -itv $PWD:/src -w /src quay.io/pypa/manylinux_2_24_x86_64:latest pipx run nox -s tests

since we've added support for running nox directly from pipx in the manylinux images, this is a handy note to add.

Describe the solution you'd like
This could either be something explicit like nox.header='...', or it could take the module docstring for the noxfile.