ownable: add a notice on renounceOwnership (#937)

* ownable: add a notice on renounceOwnership

* Apply the suggestion by @shrugs

contracts/ownership/Ownable.sol

@@ -35,6 +35,9 @@ contract Ownable {
 
   /**
    * @dev Allows the current owner to relinquish control of the contract.
+   * @notice Renouncing to ownership will leave the contract without an owner.
+   * It will not be possible to call the functions with the `onlyOwner`
+   * modifier anymore.
    */
   function renounceOwnership() public onlyOwner {
     emit OwnershipRenounced(owner);