The parameter to this function is an enum nl_item. The possible values are not given. There is an example ABDAY_1, but searching for that in the entire documentation structure yielded nothing.
Man page references the header containing recognized names:
The manifest constant names and values of item are defined in <langinfo.h>.
If I could have easily found that header file, I would not have filed this bug. What would have been most convenient would be for the text that names that hdr in the man page were to be a hyper link to the source of that file.
I tried searching the project for ABDAY_1 and got no results whatsoever
I agree that having the recognized values in the man page would be helpful, more so knowing that those don't change often (if ever).
A commit references this bug:
Date: Thu Dec 17 02:33:52 UTC 2020
New revision: 368711
nl_langinfo(3): document recognized item names
While here, remove .Tn macro usage (prompted by mandoc lint).
Reviewed by: bapt
Differential Revision: https://reviews.freebsd.org/D27606