doc: add grn_cache API
doc: add grn_cache API

TODO:
  * File list should be updated.
  * It may have some syntax errores.
    I don't generate HTML from the source. Sorry. :<

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