copy_file_range.2: Clarify that only regular files work

PR#273962 reported that copy_file_range(2) did not work
on shared memory objects and returned EINVAL.
Although the reporter felt this was incorrect, it is what
the Linux copy_file_range(2) syscall does.

Since there was no collective agreement that the FreeBSD
semantics should be changed to no longer be Linux compatible,
copy_file_range(2) still works on regular files only.

This man page update clarifies that. If, someday, copy_file_range(2)
is changed to support non-regular files, then the man page will
need to be updated to reflect that.

PR: 273962
Reviewed by: karels, pauamma_gundo.com (manpages)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D43227