Improve docs for ERC721URIStorage._burn (OpenZeppelin#3324)

Author: mcIovin

contracts/token/ERC721/extensions/ERC721URIStorage.sol

@@ -48,14 +48,9 @@ abstract contract ERC721URIStorage is ERC721 {
     }
 
     /**
-     * @dev Destroys `tokenId`.
-     * The approval is cleared when the token is burned.
-     *
-     * Requirements:
-     *
-     * - `tokenId` must exist.
-     *
-     * Emits a {Transfer} event.
+     * @dev See {ERC721-_burn}. This override additionally checks to see if a
+     * token-specific URI was set for the token, and if so, it deletes the token URI from
+     * the storage mapping.
      */
     function _burn(uint256 tokenId) internal virtual override {
         super._burn(tokenId);