docs: i2c: summary: extend introduction

- state the "official" name (I²C, not I2C, according to the spec) at
   the beginning but keep using the more practical I2C elsewhere
 - mention some known different names
 - add link to the specification document

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