Doc: Document issue with import tables

The import table does not work reliably together with re-evaluation of
routes due to recursive next hops or flowspec validation. We will at
least document that here, as import tables are completely redesigned and
this issue is fixed in BIRD 3.x branch.

doc/bird.sgml

@@ -2982,6 +2982,13 @@ be used in explicit configuration.
 	be examined later by <cf/show route/, and can be used to reconfigure
 	import filters without full route refresh. Default: off.
 
+	Note that currently the import table breaks routes with recursive
+	nexthops (e.g. ones from IBGP, see <ref id="bgp-gateway" name="gateway
+	recursive">), they are not properly updated after next hop change. For
+	the same reason, it also breaks re-evaluation of flowspec routes with
+	<ref id="bgp-validate" name="flowspec validation"> option enabled on
+	flowspec channels.
+
 	<tag><label id="bgp-export-table">export table <m/switch/</tag>
 	A BGP export table contains all routes sent to given BGP neighbor, after
 	application of export filters. It is also called <em/Adj-RIB-Out/ in BGP