doc coding-style: add reason for const
doc coding-style: add reason for const

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