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: Author: yuripv Date: Thu Dec 17 02:33:52 UTC 2020 New revision: 368711 URL: https://svnweb.freebsd.org/changeset/base/368711 Log: nl_langinfo(3): document recognized item names While here, remove .Tn macro usage (prompted by mandoc lint). PR: 251468 Reviewed by: bapt Differential Revision: https://reviews.freebsd.org/D27606 Changes: head/lib/libc/locale/nl_langinfo.3