Improve comment in regc_pg_locale.c.

Reported-by: Noah Misch <noah@leadboat.com>
Reviewed-by: Noah Misch <noah@leadboat.com>
Discussion: https://postgr.es/m/20250412123430.8c.nmisch@google.com

src/backend/regex/regc_pg_locale.c

@@ -21,22 +21,22 @@
 #include "utils/pg_locale.h"
 
 /*
- * To provide as much functionality as possible on a variety of platforms,
- * without going so far as to implement everything from scratch, we use
- * several implementation strategies depending on the situation:
+ * For the libc provider, to provide as much functionality as possible on a
+ * variety of platforms without going so far as to implement everything from
+ * scratch, we use several implementation strategies depending on the
+ * situation:
  *
  * 1. In C/POSIX collations, we use hard-wired code.  We can't depend on
  * the <ctype.h> functions since those will obey LC_CTYPE.  Note that these
  * collations don't give a fig about multibyte characters.
  *
- * 2. In the "default" collation (which is supposed to obey LC_CTYPE):
- *
- * 2a. When working in UTF8 encoding, we use the <wctype.h> functions.
+ * 2. When working in UTF8 encoding, we use the <wctype.h> functions.
  * This assumes that every platform uses Unicode codepoints directly
- * as the wchar_t representation of Unicode.  On some platforms
+ * as the wchar_t representation of Unicode.  (XXX: ICU makes this assumption
+ * even for non-UTF8 encodings, which may be a problem.)  On some platforms
  * wchar_t is only 16 bits wide, so we have to punt for codepoints > 0xFFFF.
  *
- * 2b. In all other encodings, we use the <ctype.h> functions for pg_wchar
+ * 3. In all other encodings, we use the <ctype.h> functions for pg_wchar
  * values up to 255, and punt for values above that.  This is 100% correct
  * only in single-byte encodings such as LATINn.  However, non-Unicode
  * multibyte encodings are mostly Far Eastern character sets for which the
@@ -46,14 +46,11 @@
  * the platform's wchar_t representation matches what we do in pg_wchar
  * conversions.
  *
- * 3. Here, we use the locale_t-extended forms of the <wctype.h> and <ctype.h>
- * functions, under exactly the same cases as #2.
- *
- * There is one notable difference between cases 2 and 3: in the "default"
- * collation we force ASCII letters to follow ASCII upcase/downcase rules,
- * while in a non-default collation we just let the library functions do what
- * they will.  The case where this matters is treatment of I/i in Turkish,
- * and the behavior is meant to match the upper()/lower() SQL functions.
+ * As a special case, in the "default" collation, (2) and (3) force ASCII
+ * letters to follow ASCII upcase/downcase rules, while in a non-default
+ * collation we just let the library functions do what they will.  The case
+ * where this matters is treatment of I/i in Turkish, and the behavior is
+ * meant to match the upper()/lower() SQL functions.
  *
  * We store the active collation setting in static variables.  In principle
  * it could be passed down to here via the regex library's "struct vars" data