Document that you shouldn't Q_ENUM() things outside the int range

The meta object system stores enums as signed int, probably for
performance reasons. This is good enough for about 99% of the use cases.
If you try to register larger types and then access them through the
metaobject system, you can get undefined behavior.

Task-number: QTBUG-71947
Change-Id: I16b395547c22fad10b476c2c2a0768538db0a20e
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/kernel/qobject.cpp

@@ -4338,6 +4338,12 @@ QDebug operator<<(QDebug dbg, const QObject *o)
     in a QVariant, you can convert them to strings. Likewise, passing them to
     QDebug will print out their names.
 
+    Mind that the enum values are stored as signed \c int in the meta object system.
+    Registering enumerations with values outside the range of values valid for \c int
+    will lead to overflows and potentially undefined behavior when accessing them through
+    the meta object system. QML, for example, does access registered enumerations through
+    the meta object system.
+
     \sa {Qt's Property System}
 */
 
@@ -4389,6 +4395,12 @@ QDebug operator<<(QDebug dbg, const QObject *o)
     used in a QVariant, you can convert them to strings. Likewise, passing them
     to QDebug will print out their names.
 
+    Mind that the enum values are stored as signed \c int in the meta object system.
+    Registering enumerations with values outside the range of values valid for \c int
+    will lead to overflows and potentially undefined behavior when accessing them through
+    the meta object system. QML, for example, does access registered enumerations through
+    the meta object system.
+
     \sa {Qt's Property System}
 */