Add extension support for argument count to ScriptInstance

doc/classes/ScriptExtension.xml

@@ -84,6 +84,7 @@
 			<return type="Variant" />
 			<param index="0" name="method" type="StringName" />
 			<description>
+				Return the expected argument count for the given [param method], or [code]null[/code] if it can't be determined (which will then fall back to the default behavior).
 			</description>
 		</method>
 		<method name="_get_script_method_list" qualifiers="virtual const">