Doc: Add since version for QJson method and enum

QJsonValue::toInt() introduced in 5.2, 7372c6cf9d4
QJsonDocument::JsonFormat introduced in 5.1, 4bb5566632

Task-number: QTBUG-69527
Change-Id: Idb9df959f82fe7061e0afe2109f1ca34a4451a5f
Reviewed-by: Mitch Curtis <mitch.curtis@qt.io>

src/corelib/serialization/qjsondocument.cpp

@@ -344,6 +344,7 @@ QByteArray QJsonDocument::toJson() const
 
 /*!
     \enum QJsonDocument::JsonFormat
+    \since 5.1
 
     This value defines the format of the JSON byte array produced
     when converting to a QJsonDocument using toJson().

src/corelib/serialization/qjsonvalue.cpp

@@ -558,6 +558,7 @@ bool QJsonValue::toBool(bool defaultValue) const
 }
 
 /*!
+    \since 5.2
     Converts the value to an int and returns it.
 
     If type() is not Double or the value is not a whole number,