Documentation: Add note about action 3 signals cargo types

docs/newgrf-additions.html

@@ -592,7 +592,8 @@
 	however, this applies to all signals, not only those of a particular rail type.</p>
 	<p>Variational Action 2 variables 10, 18 and 40 are available and have the same format as in <a href="https://newgrf-specs.tt-wiki.net/wiki/VariationalAction2/Railtypes">VariationalAction2/Railtypes</a>.</p>
 	<p>Rail type custom signal sprites have a higher priority than custom signal sprites for all signals as set here.</p>
-	<p>Note that this is not a generic callback, the sprite group must be assigned to ID 0 (further IDs may be allocated for other purposes in future).</p>
+	<p>Note that this is not a generic callback, the sprite group must be assigned to ID 0 (further IDs may be allocated for other purposes in future).
+	<br />Cargo type definitions are not used. Only the default set ID is used.</p>
 	<p>This is indicated by the feature name: <font face="monospace">action3_signals_custom_signal_sprites</font>, version 1</p>
 	<br />
 	<h3 id="action5">Action 14 - Type ID Mapping for Action 5</h3>