extattr_get_file(2): clarify RETURN VALUES

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.

MFC after: 1 week
MFC with: r368431, r368433, r368434
Sponsored by: DellEMC Isilon