Document "checkbox and numerical input" problem type

Background:

This task grew out of DOC-715, which adds info about setting a percentage tolerance to the "multiple choice and numerical input" problem type. When we looked into that, we realized that this "checkbox and numerical input" type also existed, but it hasn't been documented yet.

From the comments in DOC-715:

It looks like there's a problem type that doesn't have a template, and that we haven't documented. (The only place I've ever seen it mentioned is in the inputtypes.py file in GitHub.) It's a checkbox problem that also accepts numerical input, similar to the multiple choice + numerical input problem that has a template and documentation.
If you click the link to the inputtypes.py file above, you go to an example problem that's presented in the inputtypes.py file. You can paste this example problem in a problem component in Studio and it seems to work. However, it includes some XML elements (startouttext and endouttext) that we no longer use.

Question:

Do we want to publicize or document this problem type? If we do, we need to check with the TNL team to make sure it works, then create documentation. The TNL team might also want to create a template. It does seem to be a cool type of problem, but I don't know if it's a priority. It could be a good docathon project, but, again, we'd have to get input from the TNL team.