doc en: add missing ticket #1643
doc en: add missing ticket #1643

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