modules/wayland: document that scan_xml can take any protocol.

The quick example can be slightly misleading in implying that scan_xml
must use find_protocol, which is not the case. So explicitly mention it
in the scan_xml docs.

docs/markdown/Wayland-module.md

@@ -53,7 +53,8 @@ generated = wl_mod.scan_xml(
   include_core_only : true,
 )
 ```
-This function accepts one or more arguments of either string or file type.
+This function accepts one or more arguments of either string or file type, so
+it can be used in conjunction with `find_protocol` or not.
 
 It takes the following keyword arguments:
 - `public` Optional arg that specifies the scope of the generated code.