doc/guide/hooks-stat-cmds.xml
    Fixed return values

doc/guide/logging.xml
    Added entry for kea-dhcp6.stat-cmds-hooks logger

src/hooks/dhcp/stat_cmds/stat_cmds_messages.mes
src/hooks/dhcp/stat_cmds/stat_cmds_callouts.cc
    Removed extra _CMDS from logger IDs

src/hooks/dhcp/stat_cmds/tests/stat_cmds_unittest.cc
    Removed extraneous using statement;