documentation++

make it insanely clear that
- totalTime may return garbage unless its signal was emitted first
- it must always be expected to return garbage before entering Playing
state for the first time
- the value may change slightly as playback progresses
- the signal is not emitted when changing to a garbage value

note: qtphonon sets Loading as the thresh for valid totalTime which is
of course utter crap as a backend may not actually support Loading
in the sense that it actually loads the media while not playing or
buffering (vlc actually has that as only in initial buffering after
play it will calculate the overall size)