Update natspec of ltv in Pool.getUserAccountData

The description of the ltv field](https://github.com/aave/aave-v3-core/blob/27a6d5c83560694210849d4abf09a09dec8da388/contracts/interfaces/IPool.sol#L474) is vague and can be misinterpreted as the current LTV. 

To make it more clear, this could be updated to add more detail: "maximum Loan To Value of the user - weighted average of max ltv of collateral reserves"