Add locale: false custom route prefix note (#19765)

This makes sure to note how custom-routes need to be prefixed when `locale: false` is used for routes to be matched correctly. 

Closes: https://github.com/vercel/next.js/issues/19703

docs/api-reference/next.config.js/headers.md

@@ -178,7 +178,7 @@ module.exports = {
 
 ### Headers with i18n support
 
-When leveraging [`i18n` support](/docs/advanced-features/i18n-routing.md) with headers each `source` is automatically prefixed to handle the configured `locales` unless you add `locale: false` to the header:
+When leveraging [`i18n` support](/docs/advanced-features/i18n-routing.md) with headers each `source` is automatically prefixed to handle the configured `locales` unless you add `locale: false` to the header. If `locale: false` is used you must prefix the `source` with a locale for it to be matched correctly.
 
 ```js
 module.exports = {
@@ -209,6 +209,17 @@ module.exports = {
           },
         ],
       },
+      {
+        // this matches '/' since `en` is the defaultLocale
+        source: '/en',
+        locale: false,
+        headers: [
+          {
+            key: 'x-hello',
+            value: 'world',
+          },
+        ],
+      },
     ]
   },
 }

docs/api-reference/next.config.js/redirects.md

@@ -122,7 +122,7 @@ module.exports = {
 
 ### Redirects with i18n support
 
-When leveraging [`i18n` support](/docs/advanced-features/i18n-routing.md) with redirects each `source` and `destination` is automatically prefixed to handle the configured `locales` unless you add `locale: false` to the redirect:
+When leveraging [`i18n` support](/docs/advanced-features/i18n-routing.md) with redirects each `source` and `destination` is automatically prefixed to handle the configured `locales` unless you add `locale: false` to the redirect. If `locale: false` is used you must prefix the `source` and `destination` with a locale for it to be matched correctly.
 
 ```js
 module.exports = {
@@ -145,6 +145,13 @@ module.exports = {
         locale: false,
         permanent: false,
       },
+      {
+        // this matches '/' since `en` is the defaultLocale
+        source: '/en',
+        destination: '/en/another',
+        locale: false,
+        permanent: false,
+      },
     ]
   },
 }

docs/api-reference/next.config.js/rewrites.md

@@ -166,7 +166,7 @@ module.exports = {
 
 ### Rewrites with i18n support
 
-When leveraging [`i18n` support](/docs/advanced-features/i18n-routing.md) with rewrites each `source` and `destination` is automatically prefixed to handle the configured `locales` unless you add `locale: false` to the rewrite:
+When leveraging [`i18n` support](/docs/advanced-features/i18n-routing.md) with rewrites each `source` and `destination` is automatically prefixed to handle the configured `locales` unless you add `locale: false` to the rewrite. If `locale: false` is used you must prefix the `source` and `destination` with a locale for it to be matched correctly.
 
 ```js
 module.exports = {
@@ -187,6 +187,12 @@ module.exports = {
         destination: '/nl/another',
         locale: false,
       },
+      {
+        // this matches '/' since `en` is the defaultLocale
+        source: '/en',
+        destination: '/en/another',
+        locale: false,
+      },
     ]
   },
 }