doc: fix conflicting documentation for libevdev_get_event_value()

Yes, the value we return is from the currently active slot, but there are a
few niche cases where the active slot changes from what the client may think
it is. So let's call it undefined like the other half of the documentation
already does.

Fixes #20

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
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