ffmpeg looks for both .ffpreset and .avpreset files depending on whether the
-[avsf]pre or -pre option is used. Added two sections for each type of preset
including the rules according to which files are searched.

(Notably, the lookup order is swapped for avpreset files, because it first
looks for <codec>_<arg>.avpreset and then for <arg>.avpreset.)

This removes the section explaining -pre only, which was under "Examples",
where it did not really make sense.
Signed-off-by: Werner Robitza <werner.robitza@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>