libVLC: precise the documentation for chapters_descr parameters

If libvlc_media_player_get_full_chapter_descriptions is called with a
title set to -1, it uses the current title.

include/vlc/libvlc_media_player.h

@@ -1192,7 +1192,7 @@ LIBVLC_API
  * \version LibVLC 3.0.0 and later.
  *
  * \param p_mi the media player
- * \param index of the title to query for chapters
+ * \param index of the title to query for chapters (uses current title if set to -1)
  * \param address to store an allocated array of chapter descriptions
  *        descriptions (must be freed with libvlc_chapter_descriptions_release()
  *        by the caller) [OUT]