SSDM-7830: enhanced obis documentation

obis/src/python/obis/dm/checksum.py

obis/src/python/obis/dm/command_log.py

obis/src/python/obis/dm/command_result.py

obis/src/python/obis/dm/commands/clone.py

obis/src/python/obis/dm/commands/download.py

obis/src/python/obis/dm/commands/move.py

obis/src/python/obis/dm/commands/openbis_command.py

obis/src/python/obis/dm/commands/openbis_sync.py

obis/src/python/obis/dm/commands/removeref.py

obis/src/python/obis/dm/data_mgmt.py

obis/src/python/obis/dm/git.py