Document Symbian related QSettings information

There is some confusion regarding QSettings on Symbian.
One of the problems is sharing a qsettings between applications (the default) vs having it application private (more secure).
When it is not application private one of the problems is that it is not removed when the application is uninstalled.
See the linked taks for more information.

Some comments from IRC:

if you store it outside private/SID and don't create a custom deleter the data will remain
sure you can create a separate cleaner exe for each Qt app that will register all QSetting keys stored and delete them at uninstall. But isn't that a bit on heavy side ?
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it defaults to system level c:/data/.config/trolltech.conf
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QSettings offers a constructor where you can specify custom file, so if you want your settings not to persist, you can use that to create settings file in application's private dir.
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there is a platform-specific notes section that needs the locations added for Symbian. I thnk that should be enough
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QSettings doc page lists the platform specific stuff, so I guess that should include symbian