[MPlayer-DOCS] Better description of -slang

[Diego Biurrun]

Guillaume POIRIER writes:
> The description of -slang seem rather obfuscated to me... If I
> understood correctly what it does, here's a patch that'll make it
> understandable by random normal people.
> 
> What do you think?

Good idea, but the wording still needs some improvement.  I committed
a version that hopefully achieves what you intended.

Diego