doc: document the addition of the AVProbeData.mime_type field and it's implications

Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
lavf/format.c: use AVPROBE_SCORE_MIME instead of AVPROBE_SCORE_EXTENSION for matching mime types

Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>