i2c: extend documentation about retvals of master_xfer functions

It was stated how the error codes should be. It was not stated what the
regular case should return. Add this.
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>

include/linux/i2c.h

@@ -537,7 +537,8 @@ i2c_register_board_info(int busnum, struct i2c_board_info const *info,
  *
  * The return codes from the ``master_xfer{_atomic}`` fields should indicate the
  * type of error code that occurred during the transfer, as documented in the
- * Kernel Documentation file Documentation/i2c/fault-codes.rst.
+ * Kernel Documentation file Documentation/i2c/fault-codes.rst. Otherwise, the
+ * number of messages executed should be returned.
  */
 struct i2c_algorithm {
 	/*