Document tunable dev.sis.%unit.manual_pad

While I'm here Xref vlan(4) as sis(4) supports VLAN oversized
frames.

share/man/man4/sis.4

@@ -30,7 +30,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd October 28, 2006
+.Dd September 2, 2010
 .Dt SIS 4
 .Os
 .Sh NAME
@@ -145,6 +145,22 @@ SiS 630, 635, and 735 motherboard chipsets
 .It
 Soekris Engineering net45xx, net48xx, lan1621, and lan1641
 .El
+.Sh SYSCTL VARIABLES
+The following variable is available as both
+.Xr sysctl 8
+variable and
+.Xr loader 8
+tunable:
+.Bl -tag -width indent
+.It Va dev.sis.%unit.manual_pad
+This variable controls how to pad short frames for DP83815/DP83816
+controllers on the specified device.
+DP83815/DP83816 controllers are known to pad 0xFF for short frames
+which is violation of RFC 1042.
+A non-zero value have driver manually pad zeros for short frames
+with the cost of extra CPU cycles.
+The default valus is 0 to let hardware automatically pad short frames.
+.El
 .Sh DIAGNOSTICS
 .Bl -diag
 .It "sis%d: couldn't map ports/memory"
@@ -190,6 +206,7 @@ the card should be configured correctly.
 .Xr netintro 4 ,
 .Xr ng_ether 4 ,
 .Xr polling 4 ,
+.Xr vlan 4 ,
 .Xr ifconfig 8
 .Rs
 .%T SiS 900 and SiS 7016 datasheets