tx permissioning in plugin (#1744)

Signed-off-by: Sally MacFarlane <macfarla.github@gmail.com>

Author: macfarla

docs/private-networks/concepts/permissioning/plugin.md

@@ -10,12 +10,16 @@ tags:
 
 You can define complex [permissioning](index.md) solutions by building a plugin that extends Besu functionality.
 
-The plugin API provides a `PermissioningService` interface that currently supports connection permissioning and message permissioning.
+The plugin API provides a `PermissioningService` interface that currently supports connection (node) permissioning, transaction permissioning and message permissioning.
 
-## Connection permissioning
+## Connection (node) permissioning
 
 Use connection permissioning when deciding whether to restrict node access to known participants only.
 
+## Transaction permissioning
+
+Use transaction permissioning when deciding whether to restrict transaction processing based on transaction properties.
+
 ## Message permissioning
 
 Use message permissioning to propagate different types of devP2P messages to particular nodes. For example, this can be used to prevent pending transactions from being forwarded to other nodes.