Added documentation for the USB-ISS I2C functionalities.
Added the "iss_serial_baudrate_t" enum definition."
Modified the 'iss_set_mode' function.

This function can now handle I2C + serial mode.
Further renaming of mode set functions.

- Renamed the 'iss_set_mode' function to 'iss_set_i2c_mode'.
- Finalized the I2C+serial mode implementation.
- Fixed the documentation according to those changes.