Unicode cabal flags, [skip ci]

Cabal/doc/developing-packages.rst

@@ -2162,9 +2162,13 @@ Configuration Flags
 
    Flag section declares a flag which can be used in `conditional blocks`_.
 
-Flag names are case-insensitive and must match ``[a-z0-9_][a-z0-9_-]*``
+Flag names are case-insensitive and must match ``[[:alnum:]_][[:alnum:]_-]*``
 regular expression.
 
+.. note::
+
+    Hackage accepts ASCII-only flags, ``[a-zA-Z0-9_][a-zA-Z0-9_-]*`` regexp.
+
 .. pkg-field:: description: freeform
 
     The description of this flag.