[MPlayer-DOCS] document ass-force-style subopts

[compn]

are these all of the suboptions for it?
also some user asked about what color yellow would be.
so a list of colors or how to find the list of colors would be nice.

FontName
PrimaryColour
SecondaryColour
OutlineColour
BackColour
FontSize
Bold
Italic
Underline
StrikeOut
Spacing
Angle
BorderStyle
Alignment
MarginL
MarginR
MarginV
Encoding
ScaleX
ScaleY
Outline
Shadow

-compn