improve error message for conflicting parallel segments (#53803)

This is a follow-up to log both conflicting paths & a link to route group docs, which I believe is the only scenario someone could trigger this

- https://github.com/vercel/next.js/pull/53752

packages/next/src/build/webpack/loaders/next-app-loader.ts

@@ -437,12 +437,14 @@ const nextAppLoader: AppLoader = async function nextAppLoader() {
     pathname: string
   ): [string, string | string[]][] => {
     const matched: Record<string, string | string[]> = {}
+    let existingChildrenPath: string | undefined
     for (const appPath of normalizedAppPaths) {
       if (appPath.startsWith(pathname + '/')) {
         const rest = appPath.slice(pathname.length + 1).split('/')
 
         // It is the actual page, mark it specially.
         if (rest.length === 1 && rest[0] === 'page') {
+          existingChildrenPath = appPath
           matched.children = PAGE_SEGMENT
           continue
         }
@@ -464,12 +466,13 @@ const nextAppLoader: AppLoader = async function nextAppLoader() {
 
         // avoid clobbering existing page segments
         // if it's a valid parallel segment, the `children` property will be set appropriately
-        if (matched.children && matched.children !== rest[0]) {
+        if (existingChildrenPath && matched.children !== rest[0]) {
           throw new Error(
-            `/You cannot have two parallel pages that resolve to the same path. Please check ${appPath}.`
+            `You cannot have two parallel pages that resolve to the same path. Please check ${existingChildrenPath} and ${appPath}. Refer to the route group docs for more information: https://nextjs.org/docs/app/building-your-application/routing/route-groups`
           )
         }
 
+        existingChildrenPath = appPath
         matched.children = rest[0]
       }
     }

test/e2e/app-dir/parallel-routes-and-interception/parallel-routes-and-interception.test.ts

@@ -211,7 +211,7 @@ createNextDescribe(
 
           await check(
             () => next.cliOutput,
-            /You cannot have two parallel pages that resolve to the same path./i
+            /You cannot have two parallel pages that resolve to the same path\. Please check \/parallel\/\(new\)\/@baz\/nested-2\/page and \/parallel\/nested-2\/page\./i
           )
         }
         await next.stop()