docs: tweak createSubscriber explanation

[Rich-Harris]

supersedes #16310. adds a couple of useful explanatory notes

[changeset-bot]

⚠️ No Changeset found

Latest commit: f45c37f

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

[github-actions]

Playground

pnpm add https://pkg.pr.new/svelte@16398

[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-svelte-16398-svelte.vercel.app/