docs: i2c: smbus-protocol: improve DataLow/DataHigh definition

Use a more professional wording.

Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>