That manpage is an import from freebsd, not part of base -- As far as I can see on my system it's not there:

OpenBSD:

    $ man 2 sendfile
    man: No entry for sendfile in section 2 of the manual.

    $ grep -rni sendfile /usr/include | wc -l
       0

FreeBSD:

    # grep -rni sendfile /usr/include | wc -l
          30

edit: typo