docs: i2c: smbus-protocol: improve I2C Block transactions description

Clarify from the beginning what these transactions are, and specifically
how they differ from the SMBus counterparts, i.e. the lack of a Count byte.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Reviewed-by: Jean Delvare <jdelvare@suse.de>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>