Add 'available since' notes. Fixes #2054

contracts/token/ERC721/ERC721Metadata.sol

@@ -80,6 +80,8 @@ contract ERC721Metadata is Context, ERC165, ERC721, IERC721Metadata {
     * @dev Returns the base URI set via {_setBaseURI}. This will be
     * automatically added as a preffix in {tokenURI} to each token's URI, when
     * they are non-empty.
+    *
+    * _Available since v2.5.0._
     */
     function baseURI() external view returns (string memory) {
         return _baseURI;

contracts/utils/Create2.sol

@@ -8,6 +8,8 @@ pragma solidity ^0.5.0;
  *
  * See the https://eips.ethereum.org/EIPS/eip-1014#motivation[EIP] for more
  * information.
+ *
+ * _Available since v2.5.0._
  */
 library Create2 {
     /**

contracts/utils/SafeCast.sol

@@ -15,6 +15,8 @@ pragma solidity ^0.5.0;
  *
  * Can be combined with {SafeMath} to extend it to smaller types, by performing
  * all math on `uint256` and then downcasting.
+ *
+ * _Available since v2.5.0._
  */
 library SafeCast {