doc: drop improved feature because of disabling function call in output_columns
doc: drop improved feature because of disabling function call in output_columns

https://github.com/groonga/groonga/commit/9b8248c5fa36afd46007f4d2804e2007ab480005

komainu8/groonga,groonga/groonga,cosmo0920/groonga,redfigure/groonga,kenhys/groonga,kenhys/groonga,naoa/groonga,redfigure/groonga,cosmo0920/groonga,groonga/groonga,naoa/groonga,hiroyuki-sato/groonga,redfigure/groonga,cosmo0920/groonga,hiroyuki-sato/groonga,cosmo0920/groonga,komainu8/groonga,naoa/groonga,naoa/groonga,myokoym/groonga,hiroyuki-sato/groonga,groonga/groonga,groonga/groonga,kenhys/groonga,komainu8/groonga,komainu8/groonga,kenhys/groonga,groonga/groonga,naoa/groonga,cosmo0920/groonga,groonga/groonga,komainu8/groonga,naoa/groonga,komainu8/groonga,cosmo0920/groonga,hiroyuki-sato/groonga,groonga/groonga,myokoym/groonga,redfigure/groonga,kenhys/groonga,hiroyuki-sato/groonga,kenhys/groonga,myokoym/groonga,redfigure/groonga,myokoym/groonga,kenhys/groonga,hiroyuki-sato/groonga,komainu8/groonga,hiroyuki-sato/groonga,myokoym/groonga,cosmo0920/groonga,kenhys/groonga,naoa/groonga,komainu8/groonga,myokoym/groonga,redfigure/groonga,redfigure/groonga,cosmo0920/groonga,myokoym/groonga,myokoym/groonga,redfigure/groonga,naoa/groonga,groonga/groonga,hiroyuki-sato/groonga