docs: i2c: i2c-protocol: properly name start and stop conditions

In I2C there is no such thing as a "start bit" or a "stop bit". Use the
proper naming: "start condition" and "stop condition".

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