Explain non-standard encoding in LatinRules.xdy

In order to fulfill its function, LatinRules.xdy must use single,
non-standard byte characters (neither ASCII, nor multi-byte UTF-8).
To someone encountering the file without knowing its purpose (e.g.
due a post-processing raising a warning for the unrecognized encoding)
this is likely surprising, and may seem like a holdover from a time where
Unicode wasn't as universally supported.

The added comment should make clear that the file must stay as it is,
and in particular that it must not be "standardized" to UTF-8.

sphinx/texinputs/LatinRules.xdy

@@ -1,6 +1,10 @@
-;; style file for xindy
+;; Common Lisp style file for xindy
 ;; filename: LatinRules.xdy
 ;;
+;; Please note that this data file deliberately uses strings
+;; with single non-ascii bytes. This is intentional and
+;; follows the usage observed in similar xindy support files.
+;;
 ;; It is based upon xindy's files lang/general/utf8.xdy and
 ;; lang/general/utf8-lang.xdy which implement
 ;; "a general sorting order for Western European languages"