While some of the syscalls' behavior were documented and implied in the
RETURN VALUES section by earlier, e.g., the DESCRIPTION sections, as having
behavior of the other calls (`*_fd` vs `*_file` vs `*_link`), there was a lot
of implied return value behavior in the section prior to this change.
Explicitly document the syscall behavior per the current implementation in
sys/kern/vfs_extattr.c so others can better develop based on its explicit
documented behavior instead of having to digest the context of the manpage to
understand the appropriate behavior.