Documentation: i2c: Document that client auto-detection is a legacy mechanism

Class-based client auto-detection has been considered a legacy mechanism
for 10 yrs now. See commit 0c176170 ("i2c: add deprecation warning
for class based instantiation"). Change the documentation of how to
write an i2c client accordingly.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>