[mythtv] Picture Adjust Keys.txt addition

[Bruce Markey]

Ben Levitt wrote:
> A patch to keys.txt.  (I should have included this
> earlier...)
> 
> I add mention of the F key's function, and briefly
> explain how these playback settings are independent of
> the record-time settings.

Committed. I was planning to add this myself today along
with some other touch-ups but your explainations were better
than mine ;-).

Thanks,

--  bjm