docs(orderbook): fix order identifier & balance return types; document TP/SL triggers

[poszu]

Brings the orderbook precompile docs in line with the actual Clob contract interface.

Order identifier — the docs wrongly stated that orders are referenced by their submitOrder tx hash. In the contract, anything that rests in the book is keyed by a computed order_id = keccak256(abi.encode(signer, nonce, sequence)). cancel(canceledOrder), update(updatedOrder), and the getOrders(orderIds) read all take this order_id (verified: node/src/clob/mod.rs stores update.updatedOrder.into() as order_id; the contract param is named orderIds; trading::OrderId::new defines the formula). Corrected the orderbook reference page (the identifier hint block, cancel, update, and getOrders param/return), and the API-reference index's "transaction hashes as identifiers" note, which used the orderbook as its prime example — now reworded with correct examples (bridge withdrawal claim, recovery target) plus an explicit orderbook carve-out.

Balance return types — balanceOf and getBalance are documented as returns (uint256) but the contract returns int256 (signed; native USD is funding-adjusted and can be negative). Fixed both, and documented the previously-missing withdrawableBalance(token, account) read that pairs with them.

TP/SL triggers — added the TriggerType/TriggerGrouping enums and a triggers section documenting submitTrigger, cancelTrigger, and updateTrigger, including the firing-direction table (side × type → mark-price condition) and the fact that triggerOrder is the trigger's order_id (read back via ob_getTriggers), not the submitTrigger tx hash. Signatures verified identical to node/src/clob/contract.rs.

Solver-only (submitSolutions) and admin-only (createOrderBook) methods are intentionally left undocumented.