lavf: rewrite the av_interleaved_write_frame() doxy

Remove some vague / incorrect statements and make the text more specific
and less confusing.
lavf: rewrite the av_write_frame() doxy

Remove some vague / incorrect statements and make the text more specific
and less confusing.
lavf: add some basic documentation of the muxing process
lavf: extend / improve the AVFormatContext doxy