Add some comments
ha_innodb.h:
  Remove the flag HA_NOT_READ_PREFIX_LAST from ha_innodb.h: InnoDB has supported this from the start, but not tested yet


sql/ha_innodb.h:
  Remove the flag HA_NOT_READ_PREFIX_LAST from ha_innodb.h: InnoDB has supported this from the start, but not tested yet
sql/ha_innodb.cc:
  Add some comments