Man pages matching name modbus_rtu_set_rts_delay in section 3:

modbus_rtu_set_rts_delay.3.en	debian 10 / libmodbus-dev
modbus_rtu_set_rts_delay.3.en	debian 11 / libmodbus-dev
modbus_rtu_set_rts_delay.3.en	debian 12 / libmodbus-dev