Doc: Polish the description of changes to the Qt Multimedia module

Doc: Polish the description of changes to the Qt Multimedia module

- Fix errors
    - QtMultimediaKit is renamed to QMultimedia, not QtMultimedia
    - It's not necessary to explicitly convert metadata keys into
      strings -- Q_DECLARE_METADATA takes care of that
    - The introduction of MediaPlayer and VideoOutput does not affect
      existing code
- Follow module naming guidelines at
  http://qt-project.org/wiki/Spelling_Module_Names_in_Qt_Documentation
- Shorten descriptions, improve flow
- Remove trivia

Change-Id: I3ab9bb2f8f33cbea52b5993f3060d929a7e39ef1
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@digia.com>