Add brief description

doc/classes/Navigation2D.xml

@@ -1,6 +1,7 @@
 <?xml version="1.0" encoding="UTF-8" ?>
 <class name="Navigation2D" inherits="Node2D" category="Core" version="3.1">
 	<brief_description>
+		Class to assist with character navigation and pathfinding.
 	</brief_description>
 	<description>
 	</description>