doc en: add description about snippet
doc en: add description about snippet

TODO: add suitable execution example.

naoa/mroonga,yoku0825/mroonga,yoku0825/mroonga,yoku0825/mroonga,yoku0825/mroonga,mroonga/mroonga,naoa/mroonga,mroonga/mroonga,yoku0825/mroonga,naoa/mroonga,mroonga/mroonga,naoa/mroonga,naoa/mroonga,naoa/mroonga,yoku0825/mroonga,mroonga/mroonga