Doc: Fix section titles that confuse QDoc's autolinker

Fixes: QTBUG-91620
Change-Id: I7c407c7158324d1fbbeb78e47d2198e8ddf5daa0
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
(cherry picked from commit c1c61c3bb468b9db7b3bf1ca49e8b71a146ff780)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/doc/src/qt6-changes.qdoc

@@ -307,7 +307,7 @@
 
     \section1 String related classes
 
-    \section2 QStringView
+    \section2 The QStringView class
 
     Starting with Qt6 it is generally recommended to use \l QStringView over
     \c QStringRef. \l QStringView references a contiguous portion of a UTF-16
@@ -332,7 +332,7 @@
         string += ...;
     \endcode
 
-    \section2 QStringRef
+    \section2 The QStringRef class
 
     In Qt6 \l QStringRef got removed from Qt Core. To ease porting of existing
     applications without touching the whole code-base, the \c QStringRef class
@@ -394,7 +394,7 @@
 
     \section1 QFuture and Related Classes
 
-    \section2 QFuture
+    \section2 The QFuture class
 
     To avoid unintended usage of QFuture, there were some changes to
     QFuture API in Qt 6, which may introduce source compatibility breaks.
@@ -461,14 +461,14 @@
 
     \endlist
 
-    \section2 QPromise
+    \section2 The QPromise class
 
     In Qt 6, the new QPromise class should be used instead of unofficial
     QFutureInterface as a "setter" counterpart of QFuture.
 
     \section1 IO Classes
 
-    \section2 QProcess
+    \section2 The QProcess class
 
     In Qt 6, the QProcess::start() overload that interprets a single command string
     by splitting it into program name and arguments is renamed to QProcess::startCommand().
@@ -499,7 +499,7 @@
 
     \section1 Meta-Type system
 
-    \section2 QVariant
+    \section2 The QVariant class
 
     \c QVariant has been rewritten to use \c QMetaType for all of its operations. This implies
     behavior changes in a few methods:
@@ -520,7 +520,7 @@
 
     \endlist
 
-    \section2 QMetaType
+    \section2 The QMetaType class
 
     In Qt 6, registration of comparators, and \cQDebug and \QDataStream streaming operators is
     done automatically. Consequently, \c QMetaType::registerEqualsComparator(),
@@ -546,7 +546,7 @@
 
     \section1 Regular expression classes
 
-    \section2 QRegularExpression
+    \section2 The QRegularExpression class
 
     In Qt6, all methods taking the \c QRegExp got removed from our code-base.
     Therefore it is very likely that you will have to port your application or
@@ -777,7 +777,7 @@
     {QRegularExpression::UseUnicodePropertiesOption}
     pattern option.
 
-    \section2 QRegExp
+    \section2 The QRegExp class
 
     In Qt6 \l QRegExp got removed from Qt Core. If your application cannot be
     ported right now, \c QRegExp still exists in Qt5Compat to keep these