Mention somewhere that SafeIERC20 is strongly preferred

[ERC20](https://github.com/OpenZeppelin/openzeppelin-contracts/blob/master/contracts/token/ERC20/IERC20.sol) vs [SafeIERC20](https://github.com/OpenZeppelin/openzeppelin-contracts/blob/master/contracts/token/ERC20/SafeERC20.sol)

Review the repository's training material and the linked IERC20 and SafeIERC20 contracts to identify the relevant ERC20 guidance. Add a clear note that SafeIERC20 is strongly preferred, and consider the issue complete when that preference is stated in the appropriate documentation.

Written by the indexing model from the issue text.