Use a better magic value, clarify callback, thanks @abandeali1

Author: fulldecent

EIPS/eip-721.md

@@ -95,22 +95,19 @@ interface ERC721 /* is ERC165 */ {
     /// @dev Throws unless `msg.sender` is the current owner, an authorized
     ///  operator, or the approved address for this NFT. Throws if `_from` is
     ///  not the current owner. Throws if `_to` is the zero address. Throws if
-    ///  `_tokenId` is not a valid NFT. When transfer is complete, this function also
-    ///  calls `onNFTReceived` on `_to` and throws if the return value is not
-    ///  `keccak256("ERC721_ONNFTRECEIVED")`.
+    ///  `_tokenId` is not a valid NFT. When transfer is complete, this function
+    ///  checks if `_to` is a smart contract (code size > 0). If so, it calls
+    ///  `onNFTReceived` on `_to` and throws if the return value is not
+    ///  `bytes4(keccak256("onNFTReceived(address,uint256,bytes)"))`.
     /// @param _from The current owner of the NFT
     /// @param _to The new owner
     /// @param _tokenId The NFT to transfer
     /// @param data Additional data with no specified format, sent in call to `_to`
     function transferFrom(address _from, address _to, uint256 _tokenId, bytes[] data) external payable;
 	
     /// @notice Transfers the ownership of an NFT from one address to another address
-    /// @dev Throws unless `msg.sender` is the current owner, an authorized
-    ///  operator, or the approved address for this NFT. Throws if `_from` is
-    ///  not the current owner. Throws if `_to` is the zero address. Throws if
-    ///  `_tokenId` is not a valid NFT. When transfer is complete, this function also
-    ///  calls `onNFTReceived` on `_to` and throws if the return value is not
-    ///  `keccak256("ERC721_ONNFTRECEIVED")`.
+    /// @dev This works identically to the other function with an extra data parameter,
+    ///  except this function just sets data to []
     /// @param _from The current owner of the NFT
     /// @param _to The new owner
     /// @param _tokenId The NFT to transfer