Man pages matching name sd_bus_error_unix_process_id_unknown in section 3:

SD_BUS_ERROR_UNIX_PROCESS_ID_UNKNOWN.3.en	debian 10 / libsystemd-dev
SD_BUS_ERROR_UNIX_PROCESS_ID_UNKNOWN.3.en	debian 11 / libsystemd-dev
SD_BUS_ERROR_UNIX_PROCESS_ID_UNKNOWN.3.en	debian 12 / libsystemd-dev