fork(2): See "MT-Level of Libraries"
This is in the fork(2) man page:
fork1(), or forkx(). See "MT-Level of Libraries" on the attributes(5) manual page.
The problem is that in the attributes(5) man page there is no section named "MT-Level of Libraries". There is just "MT-Level" section there.
MT-Level Libraries are classified into categories that define their ability to support multiple threads. Manual pages containing functions that are of multiple or differing levels describe this in their NOTES or USAGE section.
Either (or both) fork(2) and/or attributes(5) man page should be fixed.