Doc: Update documentation about VRFs and BFD

	This is an optional description of the protocol. It is displayed as a

	part of the output of 'show protocols all' command.

	<tag><label id="proto-vrf">vrf "<m/text/"</tag>

	<tag><label id="proto-vrf">vrf "<m/text/"|default</tag>

	Associate the protocol with specific VRF. The protocol will be

	restricted to interfaces assigned to the VRF and will use sockets bound

	to the VRF. Appropriate VRF interface must exist on OS level. For kernel

	protocol, an appropriate table still must be explicitly selected by

	<cf/table/ option. Note that for proper VRF support it is necessary to

	use Linux kernel version at least 4.14, older versions have limited VRF

	implementation.

	to the VRF. A corresponding VRF interface must exist on OS level. For

	kernel protocol, an appropriate table still must be explicitly selected

	by <cf/table/ option.

	By selecting <cf/default/, the protocol is associated with the default

	VRF; i.e., it will be restricted to interfaces not assigned to any

	regular VRF. That is different from not specifying <cf/vrf/ at all, in

	which case the protocol may use any interface regardless of its VRF

	status.

	Note that for proper VRF support it is necessary to use Linux kernel

	version at least 4.14, older versions have limited VRF implementation.

	Before Linux kernel 5.0, a socket bound to a port in default VRF collide

	with others in regular VRFs. In BGP, this can be avoided by using

	<ref id="bgp-strict-bind" name="strict bind"> option.

	<tag><label id="proto-channel"><m/channel name/ [{<m/channel config/}]</tag>

	Every channel must be explicitly stated. See the protocol-specific

advanced features like the echo mode or authentication are not implemented), IP

transport for BFD as defined in <rfc id="5881"> and <rfc id="5883"> and

interaction with client protocols as defined in <rfc id="5882">.

We currently support at most one protocol instance.

<p>BFD packets are sent with a dynamic source port number. Linux systems use by

default a bit different dynamic port range than the IANA approved one

(49152-65535). If you experience problems with compatibility, please adjust

<cf>/proc/sys/net/ipv4/ip_local_port_range</cf>

<cf>/proc/sys/net/ipv4/ip_local_port_range</cf>.

<sect1>Configuration

<label id="bfd-config">

<p>Note that to use BFD for other protocols like OSPF or BGP, these protocols

also have to be configured to request BFD sessions, usually by <cf/bfd/ option.

<p>A BFD instance not associated with any VRF handles session requests from all

other protocols, even ones associated with a VRF. Such setup would work for

single-hop BFD sessions if <cf/net.ipv4.udp_l3mdev_accept/ sysctl is enabled,

but does not currently work for multihop sessions. Another approach is to

configure multiple BFD instances, one for each VRF (including the default VRF).

Each BFD instance associated with a VRF (regular or default) only handles

session requests from protocols in the same VRF.

<p>Some of BFD session options require <m/time/ value, which has to be specified

with the appropriate unit: <m/num/ <cf/s/|<cf/ms/|<cf/us/. Although microseconds

are allowed as units, practical minimum values are usually in order of tens of