It looks like FreeBSD is missing a manual page for macros defined in sys/sys/socket.h. The man page is present for example in Linux as cmsg(3) and in OpenBSD as CMSG_DATA(3). There's also a nice blog post about FreeBSD SCTP ancillary data by Tsvetomir Dimitrov.
Here's a manual page for CMSG_DATA based on the OpenBSD one. It looks like it does not need much tweaking as the presented information are true on FreeBSD as well. I've added cross references to related manpages and RFCs.
I am not sure about the example though. It does not define all the required variables. To make it less confusing for new users (such as me) I consolidated those OpenBSD examples into one working, compile-and-run example.