[PyDocs] Additional Info for getPlayingFile and RenderCapture

1) Added information in comments to be pulled into PyDocs describing the shortcomings when calling xbmc.player.getPlayingFile() for LiveTV. Specifically that pvr:// url's are not translated into usable filenames or streams.
2) Added information to xbmc.CapturePlayer regarding the functions getHeight() and getWidth() which return 0 prior to calling CapturePlayer.capture(). Also added that getAspectRatio() may be called prior to issuing a capture() request.