Don't generate empty doc sections and reduce code duplication

doc/classes/VisualShaderNodeExpression.xml

@@ -9,13 +9,9 @@
 	</description>
 	<tutorials>
 	</tutorials>
-	<methods>
-	</methods>
 	<members>
 		<member name="expression" type="String" setter="set_expression" getter="get_expression" default="&quot;&quot;">
 			An expression in Godot Shading Language, which will be injected at the start of the graph's matching shader function ([code]vertex[/code], [code]fragment[/code], or [code]light[/code]), and thus cannot be used to declare functions, varyings, uniforms, or global constants.
 		</member>
 	</members>
-	<constants>
-	</constants>
 </class>