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.

modules/gdscript/doc_classes/GDScript.xml

@@ -12,15 +12,13 @@
 	</tutorials>
 	<methods>
 		<method name="get_as_byte_code" qualifiers="const">
-			<return type="PackedByteArray">
-			</return>
+			<return type="PackedByteArray" />
 			<description>
 				Returns byte code for the script source code.
 			</description>
 		</method>
 		<method name="new" qualifiers="vararg">
-			<return type="Variant">
-			</return>
+			<return type="Variant" />
 			<description>
 				Returns a new instance of the script.
 				For example: