docs: add untrack documentation, fixing part of #14058

[Ocean-OS]

This adds documentation on untrack. It might need to be put with the svelte reference, I wasn't entirely sure on where to put it.
There is already a (rather small) definition on what it does in the reference, but I think it'd be better to have a clearer definition on what it does and where to use it in the $effect documentation.

Before submitting the PR, please make sure you do the following

• It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
• Prefix your PR title with feat:, fix:, chore:, or docs:.
• This message body should clearly illustrate what problems it solves.
• Ideally, include a test that fails without this PR but passes with it.

Tests and linting

• Run the tests with pnpm test and lint the project with pnpm lint

[changeset-bot]

⚠️ No Changeset found

Latest commit: 4e18b58

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

This PR includes no changesets

When changesets are added to this PR, you'll see the packages that this PR includes changesets for and the associated semver types

Click here to learn what changesets are, and how to add one.

Click here if you're a maintainer who wants to add a changeset to this PR

[pkg-pr-new]

pnpm add https://pkg.pr.new/svelte@14060

commit: 4e18b58

[Ocean-OS]

Just realized that this accidentally merges part of my other PR, #13978. Apologies, I will see how I could remedy this (if #13978 isn't merged soon).

[Rich-Harris]

Thank you. The documentation really belongs on the untrack function itself — $derived and $effect can link to it, but shouldn't be responsible for illustrating how it works. Opened #14085

[dummdidumm]

Closing in favor of #14085 - thank you!