i2c: Improve smbus-protocol documentation

Improve the smbus-protocol documentation file somewhat:

 - Use the names of the SMBus protocol operations (from the 2.0
   specification), not made-up-for-Linux names.

 - Add the name of the call used to execute each operation ... and
   point out that there are mismatches, where functions execute
   different protocol operations than their names specify.
   
The most confusing examples are that "Read Byte" isn't executed by
i2c_smbus_read_byte(), and that "Write Byte" isn't executed by
i2c_smbus_write_byte().  When coding, that's not as bad as it may
seem; but that case would seem to be worth fixing.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Jean Delvare <khali@linux-fr.org>