Documentation/erc721 contracts (#2218)

* Adds / Updates documentation of ERC721 contract

* Improve ERC721Burnable documentation

* Fix typo

* Revert changes on ERC721 private functions

* Add documentation to the ERC721 contract's constructor

* Add IERC721Receiver & ERC721Holder documentations

* Add references to IERC721 functions

* Add references to IERC721Metadata/Receiver

* PR fixes

* Fixes to ERC721 documentation

* Add missing fixes

Co-authored-by: Nicolás Venturo <nicolas.venturo@gmail.com>

contracts/token/ERC721/ERC721Burnable.sol

@@ -11,8 +11,11 @@ import "./ERC721.sol";
  */
 abstract contract ERC721Burnable is Context, ERC721 {
     /**
-     * @dev Burns a specific ERC721 token.
-     * @param tokenId uint256 id of the ERC721 token to be burned.
+     * @dev Burns `tokenId`. See {ERC721-_burn}.
+     *
+     * Requirements:
+     *
+     * - The caller must own `tokenId` or be an approved operator.
      */
     function burn(uint256 tokenId) public virtual {
         //solhint-disable-next-line max-line-length

contracts/token/ERC721/ERC721Holder.sol

@@ -4,7 +4,19 @@ pragma solidity ^0.6.0;
 
 import "./IERC721Receiver.sol";
 
+  /**
+   * @dev Implementation of the {IERC721Receiver} interface.
+   *
+   * Accepts all token transfers. 
+   * Make sure the contract is able to use its token with {IERC721-safeTransferFrom}, {IERC721-approve} or {IERC721-setApprovalForAll}.
+   */
 contract ERC721Holder is IERC721Receiver {
+
+    /**
+     * @dev See {IERC721Receiver-onERC721Received}.
+     *
+     * Always returns `IERC721Receiver.onERC721Received.selector`.
+     */
     function onERC721Received(address, address, uint256, bytes memory) public virtual override returns (bytes4) {
         return this.onERC721Received.selector;
     }

contracts/token/ERC721/IERC721.sol

@@ -43,7 +43,8 @@ interface IERC721 is IERC165 {
      *
      * Requirements:
      *
-     * - `from`, `to` cannot be zero.
+     * - `from` cannot be the zero address.
+     * - `to` cannot be the zero address.
      * - `tokenId` token must exist and be owned by `from`.
      * - If the caller is not `from`, it must be have been allowed to move this token by either {approve} or {setApprovalForAll}.
      * - If `to` refers to a smart contract, it must implement {IERC721Receiver-onERC721Received}, which is called upon a safe transfer.
@@ -59,7 +60,8 @@ interface IERC721 is IERC165 {
      *
      * Requirements:
      *
-     * - `from`, `to` cannot be zero.
+     * - `from` cannot be the zero address.
+     * - `to` cannot be the zero address.
      * - `tokenId` token must be owned by `from`.
      * - If the caller is not `from`, it must be approved to move this token by either {approve} or {setApprovalForAll}.
      *
@@ -115,7 +117,8 @@ interface IERC721 is IERC165 {
       *
       * Requirements:
       *
-      * - `from`, `to` cannot be zero.
+     * - `from` cannot be the zero address.
+     * - `to` cannot be the zero address.
       * - `tokenId` token must exist and be owned by `from`.
       * - If the caller is not `from`, it must be approved to move this token by either {approve} or {setApprovalForAll}.
       * - If `to` refers to a smart contract, it must implement {IERC721Receiver-onERC721Received}, which is called upon a safe transfer.

contracts/token/ERC721/IERC721Receiver.sol

@@ -9,18 +9,13 @@ pragma solidity ^0.6.0;
  */
 interface IERC721Receiver {
     /**
-     * @notice Handle the receipt of an NFT
-     * @dev The ERC721 smart contract calls this function on the recipient
-     * after a {IERC721-safeTransferFrom}. This function MUST return the function selector,
-     * otherwise the caller will revert the transaction. The selector to be
-     * returned can be obtained as `this.onERC721Received.selector`. This
-     * function MAY throw to revert and reject the transfer.
-     * Note: the ERC721 contract address is always the message sender.
-     * @param operator The address which called `safeTransferFrom` function
-     * @param from The address which previously owned the token
-     * @param tokenId The NFT identifier which is being transferred
-     * @param data Additional data with no specified format
-     * @return bytes4 `bytes4(keccak256("onERC721Received(address,address,uint256,bytes)"))`
+     * @dev Whenever an {IERC721} `tokenId` token is transferred to this contract via {IERC721-safeTransferFrom}
+     * by `operator` from `from`, this function is called.
+     *
+     * It must return its Solidity selector to confirm the token transfer.
+     * If any other value is returned or the interface is not implemented by the recipient, the transfer will be reverted.
+     *
+     * The selector can be obtained in Solidity with `IERC721.onERC721Received.selector`.
      */
     function onERC721Received(address operator, address from, uint256 tokenId, bytes calldata data)
     external returns (bytes4);