Document BPDU filter option

Disabled state is also BPDU filter

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>

man/man8/bridge.8

@@ -293,32 +293,45 @@ droot port selectio algorithms.
 
 .TP
 .BI state " STATE "
-the operation state of the port. This is primarily used by user space STP/RSTP
+the operation state of the port. Except state 0 (disabled),
+this is primarily used by user space STP/RSTP
 implementation. One may enter a lowercased port state name, or one of the
 numbers below. Negative inputs are ignored, and unrecognized names return an
 error.
 
 .B 0
-- port is DISABLED. Make this port completely inactive.
+- port is in
+.B DISABLED
+state. Make this port completely inactive. This is also called
+BPDU filter and could be used to disable STP on an untrusted port, like
+a leaf virtual devices.
 .sp
 
 .B 1
-- STP LISTENING state. Only valid if STP is enabled on the bridge. In this
+- STP
+.B LISTENING
+state. Only valid if STP is enabled on the bridge. In this
 state the port listens for STP BPDUs and drops all other traffic frames.
 .sp
 
 .B 2
-- STP LEARNING state. Only valid if STP is enabled on the bridge. In this
+- STP
+.B LEARNING
+state. Only valid if STP is enabled on the bridge. In this
 state the port will accept traffic only for the purpose of updating MAC
 address tables.
 .sp
 
 .B 3
-- STP FORWARDING state. Port is fully active.
+- STP
+.B FORWARDING
+state. Port is fully active.
 .sp
 
 .B 4
-- STP BLOCKING state. Only valid if STP is enabled on the bridge. This state
+- STP
+.B BLOCKING
+state. Only valid if STP is enabled on the bridge. This state
 is used during the STP election process. In this state, port will only process
 STP BPDUs.
 .sp