docs: clarify $effect.tracking

[sacrosanctic]

I saw the do not edit warning. I couldn't figure out where these changes are supposed to go. So I'm just soliciting feedback from the maintainers first.

link to preview: https://svelte-dev-git-preview-svelte-14821-svelte.vercel.app/docs/svelte/$effect#$effect.tracking

tracking context to reactive context

This term is not mentioned anywhere other than in $effect.tracking section, which is a cyclical definition.

Update description

Changed to align to similar features like browser.

true if the app is running in the browser.
https://svelte.dev/docs/kit/$app-environment#browser

The use of an if guard in the example

There are a lot of open/close brackets and it's hard to see where the $effect.tracking() start and end.

An illustrative example to include all places where this is active

like this one provided by #14757 (comment)

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[Rich-Harris]

preview: https://svelte-dev-git-preview-svelte-14821-svelte.vercel.app/

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