i2c: Mention functionality flags in SMBus protocol documentation

While the mapping between I2C adapter functionality flags and
i2c_smbus_*() helper functions is rather obvious, let's still document
it for clarity.

Also drop the reference to 2 command byte I2C block reads, there is no
support for that in the kernel at the moment.

Signed-off-by: Jean Delvare <khali@linux-fr.org>