docs: note effects lifecycle (#12993)

* docs: note effects lifecycle

closes #9265

* tweak

Author: dummdidumm

documentation/docs/03-runes/02-side-effects.md

@@ -33,6 +33,8 @@ To run _side-effects_ when the component is mounted to the DOM, and when values
 
 The function passed to `$effect` will run when the component mounts, and will re-run after any changes to the values it reads that were declared with `$state` or `$derived` (including those passed in with `$props`). Re-runs are batched (i.e. changing `color` and `size` in the same moment won't cause two separate runs), and happen after any DOM updates have been applied.
 
+You can place `$effect` anywhere, not just at the top level of a component, as long as it is called during component initialization (or while a parent effect is active). It is then tied to the lifecycle of the component (or parent effect) and will therefore destroy itself when the component unmounts (or the parent effect is destroyed).
+
 You can return a function from `$effect`, which will run immediately before the effect re-runs, and before it is destroyed ([demo](/#H4sIAAAAAAAAE42SzW6DMBCEX2Vl5RDaVCQ9JoDUY--9lUox9lKsGBvZC1GEePcaKPnpqSe86_m0M2t6ViqNnu0_e2Z4jWzP3pqGbRhdmrHwHWrCUHvbOjF2Ei-caijLTU4aCYRtDUEKK0-ccL2NDstNrbRWHoU10t8Eu-121gTVCssSBa3XEaQZ9GMrpziGj0p5OAccCgSHwmEgJZwrNNihg6MyhK7j-gii4uYb_YyGUZ5guQwzPdL7b_U4ZNSOvp9T2B3m1rB5cLx4zMkhtc7AHz7YVCVwEFzrgosTBMuNs52SKDegaPbvWnMH8AhUXaNUIY6-hHCldQhUIcyLCFlfAuHvkCKaYk8iYevGGgy2wyyJnpy9oLwG0sjdNe2yhGhJN32HsUzi2xOapNpl_bSLIYnDeeoVLZE1YI3QSpzSfo7-8J5PKbwOmdf2jC6JZyD7HxpPaMk93aHhF6utVKVCyfbkWhy-hh9Z3o_2nQIAAA==)).
 
 ```svelte