Update caveats for Activity rendering behavior (#8067)

Co-authored-by: Sebastian "Sebbie" Silbermann <sebastian.silbermann@vercel.com>

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.
+- An Activity that just renders text will not render anything rather than rendering hidden text, because there’s no corresponding DOM element to apply visibility changes to. For example, `<Activity mode="hidden"><ComponentThatJustReturnsText /></Activity>` will not produce any output in the DOM for `const ComponentThatJustReturnsText = () => "Hello, World!"`.
 
 ---
 
@@ -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.