add comment for non supported metapools

contracts/plugins/assets/curve/CurveStableMetapoolCollateral.sol

@@ -13,13 +13,17 @@ interface ICurveMetaPool is ICurvePool, IERC20Metadata {
  *  This plugin contract is intended for 2-fiattoken stable metapools that
  *  DO NOT involve RTokens, such as LUSD-fraxBP or MIM-3CRV.
  *
+ *  Does not support older metapools that have a separate contract for the
+ *  metapool's LP token. Metapool and LP token contracts must be the same.
+ *
  * tok = ConvexStakingWrapper(PairedUSDToken/USDBasePool)
  * ref = PairedUSDToken/USDBasePool pool invariant
  * tar = USD
  * UoA = USD
  *
  * @notice Curve pools with native ETH or ERC777 should be avoided,
  *  see docs/collateral.md for information
+ *
  */
 contract CurveStableMetapoolCollateral is CurveStableCollateral {
     using OracleLib for AggregatorV3Interface;