i2c: make kerneldoc about bus recovery more precise

"Used internally" is vague. What it actually means is that those fields
are populated by the core if valid GPIOs are provided. Change the
comments to reflect that.

Tested-by: Phil Reid <preid@electromag.com.au>
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>