Android: add note on using custom AndroidManifest.xml for permissions

Setting permissions manually require the user to point to the manually
managed AndroidManifest.xml file.

Pick-to: 6.9 6.8
Change-Id: Iae289ad32a12f869a5a0ec0261bc3e771d94a15d
Reviewed-by: Juha Vuolle <juha.vuolle@qt.io>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

src/corelib/kernel/qpermissions.cpp

@@ -116,6 +116,10 @@ Q_LOGGING_CATEGORY(lcPermissions, "qt.permissions", QtWarningMsg);
         </manifest>
     \endcode
 
+    To ensure the relevant permission backend is included with your
+    application, please \l {QT_ANDROID_PACKAGE_SOURCE_DIR}
+    {point the build system to your custom \c AndroidManifest.xml}.
+
     The relevant permission names are described in the documentation
     for each permission type.