docs: i2c: i2c-protocol: update introductory paragraph

This sentence dates back to the pre-git era and it does not look very
professional... As there is no clear definition of "finished", and given
this page is already a pretty good overview, not to mention it is not the
kernel responsibility to document the protocol in detail, let's update the
text accordingly.

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