i2c: testunit: improve documentation

Mention that new CMDs will be NACKed while the old one is still
on-going, that the I2C address parameter of READ_BYTES is 7 bit only,
and reword one paragraph to be more precise.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>