Add links to multicast and dbq plugins

docs/reference/migration/migrate_2_0/removals.asciidoc

@@ -40,6 +40,8 @@ can install the plugin with:
 ./bin/plugin install delete-by-query
 ------------------
 
+See {plugins}/plugins-delete-by-query.html for more information.
+
 ==== Multicast Discovery is now a plugin
 
 Support for multicast is very patchy. Linux doesn’t allow multicast listening on localhost,
@@ -55,6 +57,8 @@ still need to use multicast discovery, you can install the plugin with:
 ./bin/plugin install discovery-multicast
 ------------------
 
+See {plugins}/discovery-multicast.html for more information.
+
 ==== `_shutdown` API
 
 The `_shutdown` API has been removed without a replacement. Nodes should be