docs(ssr): explain server-only module reloads (fix #19114)

docs/guide/ssr.md

@@ -102,6 +102,52 @@ createServer()
 
 Here `vite` is an instance of [ViteDevServer](./api-javascript#vitedevserver). `vite.middlewares` is a [Connect](https://github.com/senchalabs/connect) instance which can be used as a middleware in any connect-compatible Node.js framework.
 
+::: tip SSR-only module updates
+By default, updating a module that is only imported by the SSR environment does not reload the page in the browser. Framework integrations usually handle this for you. For a low-level custom SSR setup, you can add a plugin that reloads the browser when an SSR-only module changes:
+
+```ts twoslash
+import type { EnvironmentModuleNode, Plugin } from 'vite'
+
+export function ssrReload(): Plugin {
+  return {
+    name: 'ssr-reload',
+    enforce: 'post',
+    hotUpdate: {
+      order: 'post',
+      handler({ modules, server, timestamp }) {
+        if (this.environment.name !== 'ssr') return
+
+        const invalidatedModules = new Set<EnvironmentModuleNode>()
+        let hasSsrOnlyModules = false
+
+        for (const mod of modules) {
+          if (mod.id == null) continue
+          const clientModule =
+            server.environments.client.moduleGraph.getModuleById(mod.id)
+          if (clientModule != null) continue
+
+          this.environment.moduleGraph.invalidateModule(
+            mod,
+            invalidatedModules,
+            timestamp,
+            true,
+          )
+          hasSsrOnlyModules = true
+        }
+
+        if (hasSsrOnlyModules) {
+          server.environments.client.hot.send({ type: 'full-reload' })
+          return []
+        }
+      },
+    },
+  }
+}
+```
+
+Add `ssrReload()` to the `plugins` array passed to `createViteServer` in the example above. See the [`hotUpdate` hook](./api-environment-plugins#the-hotupdate-hook) for details.
+:::
+
 The next step is implementing the `*` handler to serve server-rendered HTML:
 
 ```js twoslash [server.js]