Add Governor Timelock Control Warning (OpenZeppelin#3032)

Co-authored-by: Francisco Giordano <[email protected]>

Author: JulissaDantes

contracts/governance/extensions/GovernorTimelockControl.sol

@@ -16,6 +16,10 @@ import "../TimelockController.sol";
  * the assets and permissions must be attached to the {TimelockController}. Any asset sent to the {Governor} will be
  * inaccessible.
  *
+ * WARNING: Setting up the TimelockController to have additional proposers besides the governor introduces the risk that
+ * approved governance proposals could be blocked by the other proposers, effectively executing a Denial of Service attack,
+ * and therefore blocking access to governance-controlled assets.
+ *
  * _Available since v4.3._
  */
 abstract contract GovernorTimelockControl is IGovernorTimelock, Governor {