doc: Use self-closing tags for return and argument

For the time being we don't support writing a description for those, preferring
having all details in the method's description.

Using self-closing tags saves half the lines, and prevents contributors from
thinking that they should write the argument or return documentation there.

doc/classes/VisualShaderNodeInput.xml

@@ -11,8 +11,7 @@
 	</tutorials>
 	<methods>
 		<method name="get_input_real_name" qualifiers="const">
-			<return type="String">
-			</return>
+			<return type="String" />
 			<description>
 				Returns a translated name of the current constant in the Godot Shader Language. E.g. [code]"ALBEDO"[/code] if the [member input_name] equal to [code]"albedo"[/code].
 			</description>