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

@@ -5,9 +5,9 @@ pragma solidity ^0.5.0;
  * @dev Wrappers over Solidity's uintXX casting operators with added overflow
  * checks.
  *
- * Downcasting from uint256 in Solidity does not revert on overflow. This can 
+ * Downcasting from uint256 in Solidity does not revert on overflow. This can
  * easily result in undesired exploitation or bugs, since developers usually
- * assume that overflows raise errors. `SafeCast` restores this intuition by 
+ * assume that overflows raise errors. `SafeCast` restores this intuition by
  * reverting the transaction when such an operation overflows.
  *
  * Using this library instead of the unchecked operations eliminates an entire
@@ -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 {