Document off-chain calculation methods

[sambacha]

Off chain calculations

Frontends like Uniswap do calculations off chain to compute gas cost, this should be documented to see how APIs are actually used in production

see https://github.com/Uniswap/smart-order-router/blob/c63deae67dcc2113efa0fe6348b2a2ddca3e6265/src/routers/alpha-router/gas-models/v3/v3-heuristic-gas-model.ts?rgh-link-date=2022-02-15T05%3A33%3A45Z#L30-L47

/**
 * Computes a gas estimate for a V3 swap using heuristics.
 * Considers number of hops in the route, number of ticks crossed
 * and the typical base cost for a swap.
 *
 * We get the number of ticks crossed in a swap from the QuoterV2
 * contract.
 *
 * We compute gas estimates off-chain because
 *  1/ Calling eth_estimateGas for a swaps requires the caller to have
 *     the full balance token being swapped, and approvals.
 *  2/ Tracking gas used using a wrapper contract is not accurate with Multicall
 *     due to EIP-2929. We would have to make a request for every swap we wanted to estimate.
 *  3/ For V2 we simulate all our swaps off-chain so have no way to track gas used.
 */