Instant Insights: When unable to complete validation provide a filename for the unvalidated boundary (#93770)

The error message previously did not say which parallel route slot
couldn't be validated. Now it poitns to the nearest segment below the
validation boundary. The idea is you go look abive this and figure out
if it is intentional that this segment isn't rendered in the current
context. If so you can ignored the warning (it should ideally not even
be presented as a normal error) but if it is unintentional you
investigate why you aren't SSRing this slot or you mark the slot as
intentionally blocking so it doesn't get validated.

errors/unrendered-instant-segment.mdx

@@ -0,0 +1,72 @@
+---
+title: An expected segment was not rendered during instant UI validation
+---
+
+## Why This Error Occurred
+
+Next.js attempted to validate that navigation to the affected route is able to be rendered instantly without waiting on a server request or data loading.
+
+In this instance Next.js was unable to verify that the navigation would be instant because something prevented the necessary page data from rendering during validation.
+
+## Common Causes
+
+### A parent layout conditionally omits a parallel slot
+
+If a layout receives multiple slot props (e.g. `children`, `@modal`, `@sidebar`) but only renders some of them based on a condition, the omitted slot's content never renders.
+
+```tsx
+// app/dashboard/layout.tsx
+export default function Layout({
+  children,
+  modal,
+}: {
+  children: React.ReactNode
+  modal: React.ReactNode
+}) {
+  return (
+    <main>
+      {children}
+      {
+        // `modal` is only rendered conditionally.
+        showModal ? modal : null
+      }
+    </main>
+  )
+}
+```
+
+### A client component does not render its children during SSR
+
+If a client component does not render during SSR, any segments it would have rendered as children cannot be validated for instant UI.
+
+```tsx
+import dynamic from 'next/dynamic'
+
+// This component and its children will not render during SSR.
+const ClientOnly = dynamic(() => import('./my-component'), { ssr: false })
+
+export default async function Layout({ children }) {
+  // The children of this layout won't appear in the SSR HTML
+  // but can render fine on client navigation
+  return <ClientOnly>{children}</ClientOnly>
+}
+```
+
+## How to Fix It
+
+### If the missing segment is intentional
+
+If you expect this segment to sometimes not render (for example, a modal slot that only appears on certain routes), you can opt it out of instant UI validation:
+
+```tsx
+// app/dashboard/@modal/page.tsx
+export const unstable_instant = false
+
+export default function ModalPage() {
+  // ...
+}
+```
+
+### If the missing segment is unintentional
+
+Check the parent layouts above the reported segment. Make sure every layout renders its slot props (`children` and any named parallel routes). If a client component wraps the segment, ensure it renders its children during SSR.