Add comment and tests for zero address behavior in Ownable2Step.transferOwnership() (#5226)

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

contracts/access/Ownable2Step.sol

@@ -37,6 +37,8 @@ abstract contract Ownable2Step is Ownable {
     /**
      * @dev Starts the ownership transfer of the contract to a new account. Replaces the pending transfer if there is one.
      * Can only be called by the current owner.
+     *
+     * Setting `newOwner` to the zero address is allowed; this can be used to cancel an initiated ownership transfer.
      */
     function transferOwnership(address newOwner) public virtual override onlyOwner {
         _pendingOwner = newOwner;