Finish all documentation

There is still missing documentation throughout the `stellar_rust_sdk`, including:

- `offers`: OffersForAccountRequest
- `order_book`: DetailsRequest, mod.rs
- `payments`: AllPaymentsRequest, PaymentsForAccountRequest, PaymentsForLedgerRequest, PaymentForTransactionRequest, mod.rs
- `trades`: TradesForAccountRequest, TradesForLiquidityPoolRequest, TradesForOfferRequest
- `transactions`: SingleTransactionRequest, TransactionsForAccountRequest, TransactionsForLedgerRequest, TransactionsForLiquidityPoolRequest, `account_muxed` field in response
- `fee_stats`: mod.rs (prelude)
- `models`: mod.rs (prelude)

Additionally, there are some minor issues with existing documentation, including:
- `horizon_client.rs`: documentation for `get_all_offers` mentions the return type `OfferResponse`, this should be `AllOffersResponse`
- `lib.rs`: documentation for `offers` mentions "fetching all effects" in code example
- `offers`: mod.rs, update prelude documentation
- `trades`: `all_trades_request.rs`: `AssetData`: change `//` to `///`