pf.conf.5: document SCTP support

Mention SCTP in the pf.conf.5

Reviewed by: tuexen
MFC after: 3 weeks
Sponsored by: Orange Business Services
Differential Revision: https://reviews.freebsd.org/D40870

(cherry picked from commit 47d0c1fe7d3279e9d38df75cf0c359b1fbc26d5e)