Doc clarification on emission of updateTimeout() in QGeoPositionInfoSource

The error() signal is not emitted in case of an updateTimeout() signal.

Change-Id: If4fc4c723f042f2e6feed66ffac32fb857e09ae9
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Reviewed-by: Alex Blasche <alexander.blasche@digia.com>