mp3dec: fix reading the Xing tag

The quality scale field is only supposed to be present if the fourth bit
is set. In practice, lame always sets it, but other tools might not.

CC:libav-stable@libav.org
doc: reword the mp3 muxer documentation

Make it more structured.
avconv: copy stream-level side data when streamcopying
mp3enc: write full LAME frame

Most importantly, it contains the encoder delay and replaygain info.