Add access control comment for ERC721 _burn function (#3576)

* Added access control comment for ERC721 _burn function

* Update contracts/token/ERC721/ERC721.sol

Co-authored-by: Hadrien Croubois <hadrien.croubois@gmail.com>

* Update contracts/token/ERC721/ERC721.sol

Co-authored-by: Francisco <frangio.1@gmail.com>

Co-authored-by: Hadrien Croubois <hadrien.croubois@gmail.com>
Co-authored-by: Francisco <frangio.1@gmail.com>

contracts/token/ERC721/ERC721.sol

@@ -293,6 +293,7 @@ contract ERC721 is Context, ERC165, IERC721, IERC721Metadata {
     /**
      * @dev Destroys `tokenId`.
      * The approval is cleared when the token is burned.
+     * This is an internal function that does not check if the sender is authorized to operate on the token.
      *
      * Requirements:
      *