PR: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=184110
This is a first attempt at documenting the sysctl(3) MIBs belonging to sctp(4) in the sctp(4) man page. The documentation is a copy-paste of sysctl -d net.inet.sctp, with some mandoc formatting. I can't find additional documentation on these MIBs, but I figure it's better to have them in the manpage than to have the manpage pretend they don't exist at all.
Additionally:
Added subsection headers for socket options (which were already described), and aforementioned MIBs.
Added a reference to the blackhole(4) man page under the SEE ALSO section.