getvar --type input, and improve docs for command line arguments

README.adoc

@@ -11305,6 +11305,18 @@ You can also choose a different configuration file explicitly with:
 ./run --config data/config2.py
 ....
 
+Almost all options names are automatically deduced from their command line `--help` name: just replace `-` with `_`.
+
+More precisely, we use the `dest=` value of Python's link:https://docs.python.org/3/library/argparse.html[argparse module].
+
+To get a list of all global options that you can use, try:
+
+....
+./getvar --type input
+....
+
+but note that this does not include script specific options.
+
 === Build the documentation
 
 You don't need to depend on GitHub: