docs: hide per-feature impl modules from docs.rs nav (#603)

* docs: hide per-feature impl modules from docs.rs nav

Mirror the client::sync / client::r#async treatment from v3-ergonomics
§3 on the remaining three modules:

- subscriptions::sync / r#async
- market_data::historical::sync / r#async
- market_data::realtime::sync / r#async

Each parent gets `#[doc(hidden)]` on the per-feature impl mods plus a
`## Canonical paths` rustdoc section documenting where users should reach
the types from. Non-breaking: modules remain pub, just absent from
sidebar nav.

Plan: plans/one-way-to-spell.md (PR 4 of 5). No migration entry needed
per plan — close-out lands in PR 5.

* docs: add raw-identifier closing sentence to historical/realtime (/simplify)

Match the `client::mod.rs` precedent + the `subscriptions::mod.rs` block
in this PR — the raw-ident giveaway sentence applies equally to the hidden
`historical::r#async` / `realtime::r#async` modules.