add Available since 4.1 comments

contracts/token/ERC20/extensions/IERC20Metadata.sol

@@ -6,6 +6,8 @@ import "../IERC20.sol";
 
 /**
  * @dev Interface for the optional metadata functions from the ERC20 standard.
+ *
+ * _Available since v4.1._
  */
 interface IERC20Metadata is IERC20 {
     /**

contracts/token/ERC20/extensions/draft-ERC20FlashMint.sol

@@ -11,6 +11,8 @@ import "../ERC20.sol";
  *
  * Adds the {flashLoan} method, which provides flash loan support at the token
  * level. By default there is no fee, but this can be changed by overriding {flashFee}.
+ *
+ * _Available since v4.1._
  */
 abstract contract ERC20FlashMint is ERC20, IERC3156FlashLender {
     bytes32 constant private RETURN_VALUE = keccak256("ERC3156FlashBorrower.onFlashLoan");

contracts/token/ERC20/extensions/draft-ERC20Permit.sol

@@ -77,6 +77,8 @@ abstract contract ERC20Permit is ERC20, IERC20Permit, EIP712 {
 
     /**
      * @dev "Consume a nonce": return the current value and increment.
+     *
+     * _Available since v4.1._
      */
     function _useNonce(address owner) internal virtual returns (uint256 current) {
         Counters.Counter storage nonce = _nonces[owner];