Document m_catpkt(), and remove misinformation about m_cat(9).

Since m_cat() may copy data from the second mbuf chain into the last mbuf
of the first chain, it may free the first mbuf of the second chain. Thus,
the second chain is not guaranteed to be valid after m_cat() returns.

Reviewed by: glebius
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D5497