doc/muxers/hls: extend documentation

In particular, reference segment muxer and clarify difference between
segment filename number and playlist sequence number.

Should fix trac ticket #2601.
doc/muxers/segment: reference hls muxer
doc/muxers/segment: clarify ambiguos reference

Replace "The segmenter muxer" with "This muxer". The "segmenter muxer"
was never defined before, and thus it is a slightly ambiguous term.