Document untrack behavior with uninitialized async reads (NotReady propagation) #1635

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.