dvbsub: clarify a note

• clarify and update the section reference wrt. the standards document.
  • use the term 'section' rather than paragraph.
  • specify which version of the standard.
  • update the section number to that from the latest non-draft version (v1.5.1).
  • include the section title.
• add i_segment_length to the list of attributes already read, since this should clearly be included.