Correct the description of the "C" locale

It is not identical to en_US, as we have long claimed.

Fixes: QTBUG-75069
Change-Id: I236adcefdcb4120d2bf5adbcde727c5e3ca13986
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@gmx.de>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/tools/qlocale.cpp

@@ -2351,6 +2351,17 @@ QString QLocale::toString(double i, char f, int prec) const
 
     Returns a QLocale object initialized to the "C" locale.
 
+    This locale is based on en_US but with various quirks of its own, such as
+    simplified number formatting and its own date formatting. It implements the
+    POSIX standards that describe the behavior of standard library functions of
+    the "C" programming language.
+
+    Among other things, this means its collation order is based on the ASCII
+    values of letters, so that (for case-sensitive sorting) all upper-case
+    letters sort before any lower-case one (rather than each letter's upper- and
+    lower-case forms sorting adjacent to one another, before the next letter's
+    two forms).
+
     \sa system()
 */
 

src/corelib/tools/qlocale.qdoc

@@ -104,7 +104,7 @@
     This enumerated type is used to specify a language.
 
     \value AnyLanguage
-    \value C The "C" locale is identical in behavior to English/UnitedStates.
+    \value C A simplified English locale; see QLocale::c()
     \value Abkhazian
     \value Oromo
     \value Afan Obsolete, please use Oromo