Update QJsonValue(qint64) docs to reflect full precision

Amends commit 35adb74ddd915831789f0175423660f8e898942e (from 5.15) to
reflect the improved precision of 64-bit int when stored as a JSON
value.

Task-number: QTBUG-28560
Pick-to: 6.9 6.8 6.5
Change-Id: I07915bc190dd7696e58356143e8857f91e888c67
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
(cherry picked from commit 9be6e51a501dfbee1c058616b9e81b1731640e3e)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/serialization/qjsonvalue.cpp

@@ -175,9 +175,14 @@ QJsonValue::QJsonValue(int v)
 /*!
     \overload
     Creates a value of type Double, with value \a v.
-    NOTE: the integer limits for IEEE 754 double precision data is 2^53 (-9007199254740992 to +9007199254740992).
-    If you pass in values outside this range expect a loss of precision to occur.
- */
+
+    This is stored internally as a 64-bit integer, so retains its full
+    precision, as long as it is retrieved with \l toInteger(). However,
+    retrieving its value with \l toDouble() will lose precision unless the value
+    lies between ±2^53.
+
+    \sa toInteger(), toDouble()
+*/
 QJsonValue::QJsonValue(qint64 v)
     : value(v)
 {