docs: i2c: smbus-protocol: mention the repeated start condition

Sr is a repeated start, it is used in both I2C and SMBus protocols.

Provide its description and replace start ("S") conditions with repeated
start ("Sr") conditions when relevant. This allows the documentation to
match the SMBus specification available at [1].

[1] http://www.smbus.org/specs/SMBus_3_1_20180319.pdf

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>