Doc: Remove reference to Unicode 4.0

It's unclear when the reference to Unicode 4.0 standard
got added - it certainly predates the qt 4 git repository.
Anyhow, nowadays we're using later versions, and it doesn't
make much sense to highlight one specific version here.

Instead, use the correct technical term - UTF-16 code unit.
Also I added a 'correspond _to_', which sounds more common
to me.

Task-number: QTBUG-56699
Change-Id: I4bdcd9060cb2b11521638019c15ef7ab67aa768b
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>

src/corelib/tools/qstring.cpp

@@ -1200,7 +1200,7 @@ const QString::Null QString::null = { };
     \ingroup string-processing
 
     QString stores a string of 16-bit \l{QChar}s, where each QChar
-    corresponds one Unicode 4.0 character. (Unicode characters
+    corresponds to one UTF-16 code unit. (Unicode characters
     with code values above 65535 are stored using surrogate pairs,
     i.e., two consecutive \l{QChar}s.)