doc/filters: re-edit notes on filtergraph escaping

State the escaping rules more explicitly, reword various sentences and
drop confusing quoting example.

Should fix trac issue #3334.
ffmpeg: use intermediary variables in reap_filters, increase readability
configure: add missing dependency for the remuxing example
avcodec/huffyuvdec: optimize >8bps VLC reading

97479 -> 54891 decicycles

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