[mythtv] [Draft] Filter documentation.

[Robert Kulagowski]

> - The "linearblend" filter
> 
> This is the filter that is used when "Deinterlace playback" has been
> checked.  It is a simple deinterlacing filter that ignores parameters,
> and works by blending adjacent lines.  It replaces combing in interlaced
> video with a less distracting "ghost" image.

Do we need to say anything about not checking "Deinterlace Playback" if 
specifying the linearblend filter?  How does using the Deinterlace 
Playback checkbox interact with a filter chain?  Is it cumulative?