doc/muxers: fix typos in the segment chapter
doc/muxers: add a dedicated section for segment examples
doc/muxers: adopt new -codec:SPEC syntax in segment example
lavfi/avfiltergraph: check pick_format return code.
examples/muxing: improve error messages.

Illustrate the use of return values, av_err2str and
avcodec_get_name.
fate: add sub2video test.
fate: add missing bitexact flag to scale filter

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