Update migration_guide.md

runtime/reference/migration_guide.md

@@ -134,8 +134,8 @@ the default value when the option is not set has changed.
 If your project does not contain a `package.json` file, the default behavior will remain unchanged.
 
 If your project contains a `package.json` file and you do not specify the `nodeModulesDir` option, you 
-must set it to `auto` to keep the existing behavior. This is because the previous default was `true`, corresponding 
-to `auto`, while the new default in Deno 2 is `manual`. 
+must set it to `auto` to keep the default Deno 1.x auto-installing behavior. The new default in Deno 2 is `manual`,
+which expects the user to keep this directory up to date manually.
 
 See
 [Node modules directory](https://docs.deno.com/runtime/fundamentals/configuration/#node-modules-directory)