Restore lost documentation for QString::append(QStringView)

Lost in commit 2766322de37adba37e0d0d4b0054e55edff01c6c (at 6.0) as it
was buried in the midst of a lot of QStringRef documentation. This
commit restores the documentation originally added in commit
3238445b270d5fb98f4008cb10a8adcc2d35165c, but locates it among the
other QString::append() overloads and adds a \overload directive.

Change-Id: If7b98f1ae9ae8144c421a048c8a35be8474558c7
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>
(cherry picked from commit eb5f33c7a18f93002f64948a1c78b32fc678a5af)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/text/qstring.cpp

@@ -3039,6 +3039,14 @@ QString &QString::append(const QString &str)
     return *this;
 }
 
+/*!
+    \fn QString &QString::append(QStringView v)
+    \overload append()
+    \since 6.0
+
+    Appends the given string view \a v to this string and returns the result.
+*/
+
 /*!
   \overload append()
   \since 5.0