manual: Document the fact that checkwps can validate %Sx theme tags

Change-Id: Ic0dec6e36290400dbdf9b735c79f9e3fc93febdf

manual/appendix/wps_tags.tex

@@ -587,9 +587,9 @@ display cycling round the defined sublines. See
     \item It should match the \config{Source:} line in the language file.
   \end{itemize}
 
-  \note{checkwps cannot verify that the string is correct, so please check on
-    either the simulator or on target.}
-
+  \note{\fname{checkwps} can be used to verify that the translated strings
+        used in a theme are valid for a specific \dap{}. In the UI,
+        invalid strings will be replaced with a blank/empty string.}
 
 \opt{touchscreen}{
   \section{Touchscreen Areas}