docs: i2c: old-module-parameters: clarify this is for obsolete kernels

This section applies only to code for very old kernels. Avoid people
reading this unnecessarily.

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