Fix `ERC721._beforeTokenTransfer` docs (#2653)

contracts/token/ERC721/ERC721.sol

@@ -367,8 +367,7 @@ contract ERC721 is Context, ERC165, IERC721, IERC721Metadata {
      * transferred to `to`.
      * - When `from` is zero, `tokenId` will be minted for `to`.
      * - When `to` is zero, ``from``'s `tokenId` will be burned.
-     * - `from` cannot be the zero address.
-     * - `to` cannot be the zero address.
+     * - `from` and `to` are never both zero.
      *
      * To learn more about hooks, head to xref:ROOT:extending-contracts.adoc#using-hooks[Using Hooks].
      */