Doc: correct QTimer accuracy information - it depends on the timer type

This doc was probably older than the feature of timer types.

Task-number: QTBUG-33760
Change-Id: Iff4c9f9bb7287498f1ee80578b500bbde777bc78
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
Fix race condition in ~QFileInfoGatherer

tst_qfilesystemmodel was hanging regularly. In
QFileInfoGatherer::~QFileInfoGatherer() it would be stuck on wait() and
in QFileInfoGatherer::run() it would be at condition.wait(&mutex);

It looks like while abort was set to true, the while in run() had just
entered so that it would wait for condition indefinitely.

Task-number: QTBUG-29403

Change-Id: If6cebbc98ec7f54fbdf347804780bbfc5e177b3b
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>