Doc: Fix documentation of BGP gateway option

	For received routes, their <cf/gw/ (immediate next hop) attribute is

	computed from received <cf/bgp_next_hop/ attribute. This option

	specifies how it is computed. Direct mode means that the IP address from

	<cf/bgp_next_hop/ is used if it is directly reachable, otherwise the

	neighbor IP address is used. Recursive mode means that the gateway is

	computed by an IGP routing table lookup for the IP address from

	<cf/bgp_next_hop/. Note that there is just one level of indirection in

	recursive mode - the route obtained by the lookup must not be recursive

	itself, to prevent mutually recursive routes.

	<cf/bgp_next_hop/ is used and must be directly reachable. Recursive mode

	means that the gateway is computed by an IGP routing table lookup for

	the IP address from <cf/bgp_next_hop/. Note that there is just one level

	of indirection in recursive mode - the route obtained by the lookup must

	not be recursive itself, to prevent mutually recursive routes.

	Recursive mode is the behavior specified by the BGP

	standard. Direct mode is simpler, does not require any routes in a