Doc: Document BGP import table option

	for every allowed table type. Default: the same as the main table

	the channel is connected to (if eligible).

	<tag><label id="bgp-import-table">import table <m/switch/</tag>

	A BGP import table contain all received routes from given BGP neighbor,

	before application of import filters. It is also called <em/Adj-RIB-In/

	in BGP terminology. BIRD BGP by default operates without import tables,

	in which case received routes are just processed by import filters,

	accepted ones are stored in the master table, and the rest is forgotten.

	Enabling <cf/import table/ allows to store unprocessed routes, which can

	be examined later by <cf/show route/, and can be used to reconfigure

	import filters without full route refresh. Default: off.

	<tag><label id="bgp-secondary">secondary <m/switch/</tag>

	Usually, if an export filter rejects a selected route, no other route is

	propagated for that network. This option allows to try the next route in