Finish all documentation

[kevin-pease]

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 ///