add detail to deprecation notice

contracts/token/ERC721/ERC721.sol

@@ -317,7 +317,7 @@ contract ERC721 is Context, ERC165, IERC721 {
      * @dev Internal function to invoke {IERC721Receiver-onERC721Received} on a target address.
      * The call is not executed if the target address is not a contract.
      *
-     * This function is deprecated.
+     * This is an internal detail of the `ERC721` contract and its use is deprecated.
      * @param from address representing the previous owner of the given token ID
      * @param to target address that will receive the tokens
      * @param tokenId uint256 ID of the token to be transferred