The error(), finished() and canceled() signals notify the user about
internal state changes. Signal handlers may call back into the same
class instance that just emitted the signals. If the instance did not
cleanup its internal state then the sudden reentry can cause weird
behavior.

Adds a comment that the pendingCancel & pendingStart behavior is
to be removed in Qt6. It has little advantage and cuases lots of
headaches. Unfortunately all backends exhibit the behavior and
therefore every backend has to change accordingly.

Task-number: QTBUG-51307
Change-Id: Ia5bf9efd0ed27e015361b10499ced069d16a9c93
Reviewed-by: Christian Kandeler <christian.kandeler@theqtcompany.com>