Merge commit '18e6f087'

* commit '18e6f087':
  img2: document the options available
  hls: improve options description
  hls: use a meaningful long name
  hls: add start_number option
  h264: check for invalid zeros_left before writing

Conflicts:
	doc/demuxers.texi
	doc/muxers.texi

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