added feature availability info to documentation

contracts/utils/Address.sol

@@ -36,6 +36,8 @@ library Address {
      *
      * NOTE: This is a feature of the next version of OpenZeppelin Contracts.
      * @dev Get it via `npm install @openzeppelin/contracts@next`.
+     *
+     * _Available since v2.4.0._
      */
     function toPayable(address account) internal pure returns (address payable) {
         return address(uint160(account));
@@ -56,6 +58,8 @@ library Address {
      * taken to not create reentrancy vulnerabilities. Consider using
      * {ReentrancyGuard} or the
      * https://solidity.readthedocs.io/en/v0.5.11/security-considerations.html#use-the-checks-effects-interactions-pattern[checks-effects-interactions pattern].
+     *
+     * _Available since v2.4.0._
      */
     function sendValue(address payable recipient, uint256 amount) internal {
         require(address(this).balance >= amount, "Address: insufficient balance");