i2c: update i2c_algorithm documentation

Add some kerneldoc style documentaton to the i2c_algorithm
structure, and point the master_xfer return codes at the
right place in Documentation/i2c/fault_codes

Signed-off-by: Ben Dooks <[e-mail]>
Signed-off-by: Wolfram Sang <[e-mail]>

include/linux/i2c.h

···
 }
 #endif /* I2C_BOARDINFO */
 
-/*
+/**
+ * struct i2c_algorithm - represent I2C transfer method
+ * @master_xfer: Issue a set of i2c transactions to the given I2C adapter
+ *   defined by the msgs array, with num messages available to transfer via
+ *   the adapter specified by adap.
+ * @smbus_xfer: Issue smbus transactions to the given I2C adapter. If this
+ *   is not present, then the bus layer will try and convert the SMBus calls
+ *   into I2C transfers instead.
+ * @functionality: Return the flags that this algorithm/adapter pair supports
+ *   from the I2C_FUNC_* flags.
+ *
  * The following structs are for those who like to implement new bus drivers:
  * i2c_algorithm is the interface to a class of hardware solutions which can
  * be addressed using the same bus algorithms - i.e. bit-banging or the PCF8584
  * to name two of the most common.
+ *
+ * The return codes from the @master_xfer field should indicate the type of
+ * error code that occured during the transfer, as documented in the kernel
+ * Documentation file Documentation/i2c/fault-codes.
  */
 struct i2c_algorithm {
 	/* If an adapter algorithm can't do I2C-level access, set master_xfer