docs: i2c: summary: be clearer with 'controller/target' and 'adapter/client' pairs

This not only includes rewording, but also where to put which emphasis
on terms in this document.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Easwar Hariharan <eahariha@linux.microsoft.com>
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>