update comment

types/shared.d.ts

@@ -7,7 +7,11 @@ export type Awaitable<T> = T | PromiseLike<T>
 
 export interface PageData {
   relativePath: string
-  filePath: string // differs from relativePath in case of path rewrites
+  /**
+   * differs from relativePath in case of path rewrites
+   * empty string if the page is virtual (e.g. 404 page)
+   */
+  filePath: string
   title: string
   titleTemplate?: string | boolean
   description: string