Apologies for the delay in answering, Emily.  And, unfortunately, I don't have a good answer.  If you would like to write a patch, I would suggest just sticking the documentation wherever you think is best and make sure there is a link to it from the Symbols index page.  Whoever commits your patch will either agree with your placement or have a better idea of where to put it, and will either move it themselves or work with you to move it.

Here are a few possible places it could go, off the top of my head and without really looking:
- in the pydoc docs
- in the help() builtin docs
- in or near the inspect.Signature docs
- in the Clinic HOWTO
- somewhere in the tutorial (though I'm not sure where would be good)