Note that sys/systm.h is special too

If you need / want to includerd sys/systm.h, it has to be just after
param.h/types.h. Document this existing practice. Not all kernel files
include systm.h, but when you do, it should be done out of order.

Reviewed by: vangyzen, kib, emaste
Differential Review: https://reviews.freebsd.org/D26981

share/man/man9/style.9

@@ -25,7 +25,7 @@
 .\"	From: @(#)style	1.14 (Berkeley) 4/28/95
 .\" $FreeBSD$
 .\"
-.Dd July 16, 2020
+.Dd October 28, 2020
 .Dt STYLE 9
 .Os
 .Sh NAME
@@ -156,9 +156,13 @@ includes
 .In sys/types.h ;
 do not include both.
 .Pc
+Next, include
+.In sys/systm.h ,
+if needed.
 The remaining kernel headers should be sorted alphabetically.
 .Bd -literal
 #include <sys/types.h>	/* Non-local includes in angle brackets. */
+#include <sys/systm.h>
 #include <sys/endian.h>
 #include <sys/lock.h>
 #include <sys/queue.h>