docs(typed-fault): batch the epic #206 review doc nits onto one car

[mfw78]

What

A single top-of-train car collecting eleven doc and comment rewords raised in the epic #206 review. Every change is rustdoc or comment only; no code behaviour moves.

• nexum-sdk host.rs, Fault::RateLimited rustdoc: say the backoff guidance may be present, matching the Option<u64> field (origin feat(wit-abi): add the fault vocabulary and SDK HostFault surface #213).
• nexum-sdk host.rs, Fault enum rustdoc: drop the restatement of the strum snake_case and #[non_exhaustive] attributes, keeping only that #[non_exhaustive] forces a wildcard match arm (origin feat(wit-abi): add the fault vocabulary and SDK HostFault surface #213).
• nexum-runtime host/error.rs, From<StorageError> rustdoc: drop the signature restatement, keep the reason the subsystem tag is omitted (origin feat(host): switch the generic interfaces to fault end to end #215).
• nexum-runtime host/error.rs, the i32::try_from saturation comment: a JSON-RPC code outside i32 is a spec violation clamped to -32603, rather than "never seen for real" (origin feat(chain): typed chain-error with an rpc case carrying revert bytes #214).
• nexum-runtime host/error.rs, the transport-level comment: keep only why a guest can tell a revert apart from an unreachable node (origin feat(chain): typed chain-error with an rpc case carrying revert bytes #214).
• shepherd-cow-host ext_cow.rs, orderbook_error_to_wit rustdoc: cowprotocol::Error is broader than a typed order submission, so map it to WIT form plainly (origin feat(cow-ext): typed cow-api-error owned by the extension package #216).
• shepherd-cow-host ext_cow.rs, cow_error_to_wit rustdoc: tighten "non-2xx orderbook reply" to a transport-layer HTTP failure and point the links at the real generated type rather than the local alias (origin feat(cow-ext): typed cow-api-error owned by the extension package #216).
• nexum-runtime host/impls/chain.rs, request_batch rustdoc: qualify "leaving its neighbours intact" as a property of this impl's independent per-entry fold, not a WIT-level contract (origin fix(runtime): typed-fault train follow-ups #218).
• shepherd-sdk cow/error.rs, OrderRejection.data rustdoc: the host serialises an already-parsed Value, so say "serialised to a JSON string via serde_json::Value::to_string" rather than "re-encodes" (origin fix(runtime): whole-train hardening for the typed-fault stack #219).
• shepherd-sdk lib.rs prelude bullet: drop "orderbook" to match prelude.rs, whose module doc no longer exposes the orderbook surface (origin fix(runtime): whole-train hardening for the typed-fault stack #219).
• nexum-runtime host/error.rs, fault_label rustdoc: drop the trailing clause that restates the preceding "Mirrors the SDK HostFault::label vocabulary" (origin feat(host): exports return fault and the host-error envelope dies #217).

Why

These regions are review hotspots already rewritten across the typed-fault stack, so the nits are collected onto one top car to avoid rippling every lower car in the train. Each reword lands on the tip's final content, located by content rather than by the reviewers' stale line numbers.

Testing

Per touched crate (nexum-sdk, nexum-runtime, shepherd-cow-host, shepherd-sdk) under the project nix shell: cargo fmt -p <crate> -- --check on the edited files, cargo clippy -p <crate> --all-targets -- -D warnings, and RUSTDOCFLAGS="-D warnings" cargo doc -p <crate> --no-deps (also --document-private-items for shepherd-cow-host to confirm the retargeted intra-doc link resolves). All clean.

AI Assistance

AI Assistance: Claude Code (Opus 4.8) used for the doc rewords and PR authoring.

[jean-neiverth]

Doc rewrites follow house style. No concerns.