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/Material.xml

@@ -12,8 +12,7 @@
 	</tutorials>
 	<methods>
 		<method name="inspect_native_shader_code">
-			<return type="void">
-			</return>
+			<return type="void" />
 			<description>
 			</description>
 		</method>