Add more info to the dosc

Author: beerose

website/src/pages/plugins/presets/preset-client.mdx

@@ -527,7 +527,9 @@ const nextConfig = {
 
 Note that you will need to provide the `artifactDirectory` path that should be the same as the one configured in your `codegen.ts`
 
-### DocumentMode
+## DocumentMode
+
+### Optimizing bundle size with a string DocumentMode
 
 The `DocumentMode` option can be used to control how the plugin will generate the document nodes.
 
@@ -605,3 +607,7 @@ fetch('https:<your-graphql-api>', {
   })
 })
 ```
+
+### When to use a string DocumentMode?
+
+The `string` DocumentMode is useful when you want to reduce the bundle size of your application. You will get string literals instead of typed ASTs on the client.