Doc: Update BGP documentation

using the following configuration parameters:

<descrip>

	<tag><label id="bgp-local">local [<m/ip/] as <m/number/</tag>

	<tag><label id="bgp-local">local [<m/ip/] [port <m/number/] [as <m/number/]</tag>

	Define which AS we are part of. (Note that contrary to other IP routers,

	BIRD is able to act as a router located in multiple AS'es simultaneously,

	but in such cases you need to tweak the BGP paths manually in the filters

	to get consistent behavior.) Optional <cf/ip/ argument specifies a source

	address, equivalent to the <cf/source address/ option (see below). This

	parameter is mandatory.

	address, equivalent to the <cf/source address/ option (see below).

	Optional </cf/port> argument specifies the local BGP port instead of

	standard port 179. The parameter may be used multiple times with

	different sub-options (e.g., both <cf/local 10.0.0.1 as 65000;/ and

	<cf/local 10.0.0.1; local as 65000;/ are valid). This parameter is

	mandatory.

	<tag><label id="bgp-neighbor">neighbor [<m/ip/] [port <m/number/] [as <m/number/]</tag>

	Define neighboring router this instance will be talking to and what AS

	it is located in. In case the neighbor is in the same AS as we are, we

	automatically switch to iBGP. Optionally, the remote port may also be

	specified. The parameter may be used multiple times with different

	sub-options (e.g., both <cf/neighbor 10.0.0.1 as 65000;/ and

	<cf/neighbor 10.0.0.1; neighbor as 65000;/ are valid). This parameter is

	mandatory.

	specified. Like <cf/local/ parameter, this parameter may also be used

	multiple times with different sub-options. This parameter is mandatory.

	<tag><label id="bgp-iface">interface <m/string/</tag>

	Define interface we should use for link-local BGP IPv6 sessions.