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.
@@ -63,7 +64,7 @@ It takes the following keyword arguments:
 - `server` Optional arg that specifies if server side header file is
   generated. The default is false.
 - `include_core_only` Optional arg that specifies that generated headers only include
-  `wayland-<client|server>-core.h` instead of `wayland-<client|server>.h`. 
+  `wayland-<client|server>-core.h` instead of `wayland-<client|server>.h`.
   The default is true. Since *0.64.0*
 
 **Returns**: a list of [[@custom_tgt]] in the order source, client side header,