Update caveats for Activity rendering behavior

Clarify behavior of Activity with text nodes in caveats section.

Author: TkDodo

src/content/reference/react/Activity.md

@@ -48,6 +48,7 @@ In this way, Activity can be thought of as a mechanism for rendering "background
 #### Caveats {/*caveats*/}
 
 - If an Activity is rendered inside of a [ViewTransition](/reference/react/ViewTransition), and it becomes visible as a result of an update caused by [startTransition](/reference/react/startTransition), it will activate the ViewTransition's `enter` animation. If it becomes hidden, it will activate its `exit` animation.
+- If an Activity has a text node (plain text) as its child instead of an element, it won’t be rendered at all rather than hidden, because there’s no corresponding DOM node to apply visibility changes to.
 
 ---
 
@@ -1248,4 +1249,4 @@ When an `<Activity>` is "hidden", all its children's Effects are cleaned up. Con
 
 If you're relying on an Effect mounting to clean up a component's side effects, refactor the Effect to do the work in the returned cleanup function instead.
 
-To eagerly find problematic Effects, we recommend adding [`<StrictMode>`](/reference/react/StrictMode) which will eagerly perform Activity unmounts and mounts to catch any unexpected side-effects. 
+To eagerly find problematic Effects, we recommend adding [`<StrictMode>`](/reference/react/StrictMode) which will eagerly perform Activity unmounts and mounts to catch any unexpected side-effects.