[18] Add docs for useInsertionEffect (#4486)

Author: rickhanlonii

content/docs/hooks-reference.md

@@ -599,8 +599,8 @@ const selectedField = useSyncExternalStore(
 useInsertionEffect(didUpdate);
 ```
 
-TODO: description
+The signature is identical to `useEffect`, but it fires synchronously _before_ all DOM mutations. Use this to inject styles into the DOM before reading layout in [`useLayoutEffect`](#uselayouteffect). Since this hook is limited in scope, this hook does not have access to refs and cannot schedule updates.
 
 > Note:
-> 
-> TODO: no refs
+>
+> `useInsertionEffect` should be limited to css-in-js library authors. Prefer [`useEffect`](#useeffect) or [`useLayoutEffect`](#uselayouteffect) instead.