Fix/add comment erc721 burnable #1464 (#1469)

* fix Add a comment to ERC721Enumerable #1465

* fix Add comments to ERC721Burnable #1464

contracts/token/ERC721/ERC721Burnable.sol

@@ -2,7 +2,16 @@ pragma solidity ^0.4.24;
 
 import "./ERC721.sol";
 
+/**
+ * @title ERC721 Burnable Token
+ * @dev ERC721 Token that can be irreversibly burned (destroyed).
+ */
 contract ERC721Burnable is ERC721 {
+
+  /**
+   * @dev Burns a specific ERC721 token.
+   * @param tokenId uint256 id of the ERC721 token to be burned.
+   */
   function burn(uint256 tokenId)
     public
   {

contracts/token/ERC721/ERC721Enumerable.sol

@@ -4,6 +4,10 @@ import "./IERC721Enumerable.sol";
 import "./ERC721.sol";
 import "../../introspection/ERC165.sol";
 
+/**
+ * @title ERC-721 Non-Fungible Token with optional enumeration extension logic
+ * @dev See https://github.com/ethereum/EIPs/blob/master/EIPS/eip-721.md
+ */
 contract ERC721Enumerable is ERC165, ERC721, IERC721Enumerable {
   // Mapping from owner to list of owned token IDs
   mapping(address => uint256[]) private _ownedTokens;