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

share/man/man9/Makefile

@@ -1053,6 +1053,7 @@ MLINKS+=\
 	mbuf.9 m_append.9 \
 	mbuf.9 m_apply.9 \
 	mbuf.9 m_cat.9 \
+	mbuf.9 m_catpkt.9 \
 	mbuf.9 MCHTYPE.9 \
 	mbuf.9 MCLGET.9 \
 	mbuf.9 m_collapse.9 \

share/man/man9/mbuf.9

@@ -24,7 +24,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd March 11, 2015
+.Dd February 29, 2016
 .Dt MBUF 9
 .Os
 .\"
@@ -122,6 +122,8 @@
 .Fc
 .Ft void
 .Fn m_cat "struct mbuf *m" "struct mbuf *n"
+.Ft void
+.Fn m_catpkt "struct mbuf *m" "struct mbuf *n"
 .Ft u_int
 .Fn m_fixhdr "struct mbuf *mbuf"
 .Ft void
@@ -907,12 +909,26 @@ to
 Both
 .Vt mbuf chains
 must be of the same type.
-.Fa N
-is still valid after the function returned.
-.Sy Note :
-It does not handle
-.Dv M_PKTHDR
-and friends.
+.Fa n
+is not guaranteed to be valid after
+.Fn m_cat
+returns.
+.Fn m_cat
+does not update any packet header fields or free mbuf tags.
+.\"
+.It Fn m_catpkt m n
+A variant of
+.Fn m_cat
+that operates on packets.
+Both
+.Fa m
+and
+.Fa n
+must contain packet headers.
+.Fa n
+is not guaranteed to be valid after
+.Fn m_catpkt
+returns.
 .\"
 .It Fn m_split mbuf len how
 Partition an