docs/swr: better used_channel_count docs

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
doc/swr: Mention that the internal sample format is choosen automatically if not set

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
doc/swr: try to fix non english "inclusively included between"

Googling this term has 9 hits, 7 from ffmpeg

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
doc/swr: docu lfe mix level unit & range

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
doc/swr: typo fix

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
doc/swr: elaborate on what the LFE mix level is.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>