i2c: added docs to the I2C header.

Documented all entries in the I2C main header;

	* structures:
		- i2c_bus
		- i2c_client
		- i2c_msg
		- i2c_device_info

	* inline functions
		- i2c_create_info
		- i2c_destroy_info

And various other entries.

Signed-off-by: Michel Megens <dev@michelmegens.net>