docs: i2c: summary: rewrite the "terminology" section

This section, partly dating back to the pre-git era, is somewhat
unclear and partly incorrect. Rewrite it almost completely including a
reference figure, concise but precise definition of each term and the
paths where drivers are found. Particular care has been put in clarifying
the relation between adapter and algorithm, which has no correspondence
in the I2C spec terminology.

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