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/Sprite2D.xml

@@ -11,8 +11,7 @@
 	</tutorials>
 	<methods>
 		<method name="get_rect" qualifiers="const">
-			<return type="Rect2">
-			</return>
+			<return type="Rect2" />
 			<description>
 				Returns a [Rect2] representing the Sprite2D's boundary in local coordinates. Can be used to detect if the Sprite2D was clicked. Example:
 				[codeblocks]
@@ -41,10 +40,8 @@
 			</description>
 		</method>
 		<method name="is_pixel_opaque" qualifiers="const">
-			<return type="bool">
-			</return>
-			<argument index="0" name="pos" type="Vector2">
-			</argument>
+			<return type="bool" />
+			<argument index="0" name="pos" type="Vector2" />
 			<description>
 				Returns [code]true[/code], if the pixel at the given position is opaque and [code]false[/code] in other case.
 				[b]Note:[/b] It also returns [code]false[/code], if the sprite's texture is [code]null[/code] or if the given position is invalid.