Document q(u)int128 and QT_SUPPORTS_INT128

[ChangeLog][QtCore] Added qint128 and quint128 typedefs on platforms
that support them.

Fixes: QTBUG-116925
Change-Id: Ibd55cb0b1931b48a91598d3165e5911e3a4079f2
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
(cherry picked from commit 454636afec1be8d4483b65ae564487eca9734ffd)
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>
(cherry picked from commit 66d10dc25369bfb03fb526ada97394b8af114039)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/global/qtypes.cpp

@@ -144,6 +144,39 @@ QT_BEGIN_NAMESPACE
     \sa Q_UINT64_C(), qint64, qulonglong
 */
 
+/*!
+    \typedef qint128
+    \relates <QtTypes>
+    \since 6.6
+
+    Typedef for \c{__int128} on platforms that support it (Qt defines the macro
+    \l QT_SUPPORTS_INT128 if this is the case).
+
+    \sa quint128, QT_SUPPORTS_INT128
+*/
+
+/*!
+    \typedef quint128
+    \relates <QtTypes>
+    \since 6.6
+
+    Typedef for \c{unsigned __int128} on platforms that support it (Qt defines
+    the macro \l QT_SUPPORTS_INT128 if this is the case).
+
+    \sa qint128, QT_SUPPORTS_INT128
+*/
+
+/*!
+    \macro QT_SUPPORTS_INT128
+    \relates <QtTypes>
+    \since 6.6
+
+    Qt defines this macro as well as the \l qint128 and \l quint128 types if
+    the platform has support for 128-bit integer types.
+
+    \sa qint128, quint128
+*/
+
 /*!
     \typedef qintptr
     \relates <QtTypes>