Clarify QQuickItem::map{To,From}Item when passed a nullptr

Passing nullptr when mapping to/from an item results in mapping
relative to the scene's coordinate system, and this is also what
the "Scene Coordinates" section of "Concepts - Visual Coordinates
in Qt Quick" documents.

Avoid the ambiguous term "root QML view", which may be mistaken for
the top level window in a child-window scenario.

Pick-to: 6.6 6.5
Change-Id: Ibd386ec8248f5e398b04248d15c7cf48bd46c090
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@qt.io>
(cherry picked from commit 2ea08a9918)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/quick/items/qquickitem.cpp

@@ -4682,7 +4682,7 @@ static bool unwrapMapFromToFromItemArgs(QQmlV4Function *args, const QQuickItem *
     \input item.qdocinc mapping
 
     If \a item is a \c null value, this maps the point or rect from the coordinate system of
-    the root QML view.
+    the \l{Scene Coordinates}{scene}.
 
     The versions accepting point and rect are since Qt 5.15.
 */
@@ -4740,7 +4740,7 @@ QTransform QQuickItem::itemTransform(QQuickItem *other, bool *ok) const
     \input item.qdocinc mapping
 
     If \a item is a \c null value, this maps the point or rect to the coordinate system of the
-    root QML view.
+    \l{Scene Coordinates}{scene}.
 
     The versions accepting point and rect are since Qt 5.15.
 */