docs: i2c: smbus-protocol: enable kernel-doc function syntax

Hyperlinks from function names are not generated in headings. Move them in
the plain text so they are rendered as clickable hyperlinks.

While there also remove an unneeded colon in a heading.

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