Document untrack behavior with uninitialized async reads (NotReady propagation)

From solidjs/solid#3007 (reported by @mizulu).

The untrack reference page (https://v2.solidjs.com/reference/solid-js/reactivity/untrack) should document how untrack interacts with async reads in 2.0:

• untrack stops dependency tracking — reads inside it do not subscribe the surrounding computation.
• It does not opt out of async settlement. Reading an async source that is not yet ready inside untrack still throws NotReadyError, which propagates to the owning computation: the owner suspends (participates in Loading boundaries) and re-runs once the source first resolves.

In other words, untrack(() => getColor()) where getColor is a pending async memo will still cause the owner to re-run when getColor first settles — the read is untracked, but the not-ready suspension is part of async graph resolution, not tracking.

The page should spell out this distinction (tracking vs. settlement) and show the pattern for a genuinely non-suspending read of a possibly-pending source (e.g. checking readiness with isPending/latest as appropriate) so users aren't surprised that untrack alone doesn't provide a fallback-value escape hatch. A fallback-value overload for untrack was considered in solidjs/solid#3007 and rejected — the docs are the fix.

Issue にリンクされている untrack のリファレンスページから始め、そこに記載されている非同期読み取りの動作を solidjs/solid#3007 のコンテキストと併せて確認してください。依存関係の追跡と NotReady の確定を区別し、owner のサスペンドと再実行を説明し、isPending/latest を使った実際にはサスペンドしない準備状態のチェックを示すようにページを更新してください。