doc: improve QPlatformIntegration::possibleKeys() documentation

- There is no need to mention qkeymapper, which is an internal
implementation detail.

- Describe the encoding of int.

- Add a note that calling possibleKeys() outside key event
handler context is not valid.

Change-Id: Ife9b7d1496f04b5a433ed2d56f29c4f01f174441
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>

src/gui/kernel/qplatformintegration.cpp

@@ -446,12 +446,13 @@ Qt::KeyboardModifiers QPlatformIntegration::queryKeyboardModifiers() const
 
 /*!
   Should be used to obtain a list of possible shortcuts for the given key
-  event. As that needs system functionality it cannot be done in qkeymapper.
+  event. Shortcuts should be encoded as int(Qt::Key + Qt::KeyboardModifiers).
 
-  One example for more than 1 possibility is the key combination of Shift+5.
+  One example for more than one possibility is the key combination of Shift+5.
   That one might trigger a shortcut which is set as "Shift+5" as well as one
-  using %. These combinations depend on the currently set keyboard layout
-  which cannot be obtained by Qt functionality.
+  using %. These combinations depend on the currently set keyboard layout.
+
+  \note This function should be called only from key event handlers.
 */
 QList<int> QPlatformIntegration::possibleKeys(const QKeyEvent *) const
 {