doc en: add note about limitation of DATE, DATETIME, TIMESTAMP
doc en: add note about limitation of DATE, DATETIME, TIMESTAMP

TODO: confirm the results of execution example about TIMESTAMP

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