docs: i2c: rename sections so the overall picture is clearer

Some of the section names are not very clear. Reading those names in the
index.rst page does not help much in grasping what the content is supposed
to be.

Rename those sections to clarify their content, especially when reading
the index page.

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