Doc: update QJsonObject::operator[] non-const to say it creates key

the non-const QJsonArray::operator[] requires a valid index and all the
const operator[] say that it returns Undefined if the key or index
doesn't exist. This is the exception.

Task-number: QTBUG-39864
Change-Id: Iaa63461109844e978376fffd15f9716f0cc66cca
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

src/corelib/serialization/qjsonobject.cpp

@@ -401,7 +401,9 @@ QJsonValue QJsonObject::operator [](const QString &key) const
 
 #if QT_STRINGVIEW_LEVEL < 2
 /*!
-    Returns a reference to the value for \a key.
+    Returns a reference to the value for \a key. If there is no value with key
+    \a key in the object, one is created with a QJsonValue::Null value and then
+    returned.
 
     The return value is of type QJsonValueRef, a helper class for QJsonArray
     and QJsonObject. When you get an object of type QJsonValueRef, you can