docs(recipe: expose isServer value to reuse across app (t3-oss#258)

docs/src/app/docs/recipes/page.mdx

@@ -82,3 +82,22 @@ const config: StorybookConfig = {
 
 export default config;
 ```
+
+## isServer
+
+The `shared` section can be used to define environment variables that are accessible
+in both client and server contexts. This is particularly useful for distinguishing 
+between server and client environments without repeating checks throughout your application.
+
+```ts
+export const env = createEnv({
+  shared: {
+    IS_SERVER: z.boolean().default(false),
+  },
+  runtimeEnv: {
+    IS_SERVER: typeof window === "undefined",
+  }
+})
+```
+
+See also way to [customize `isServer` value used by the library itself](../customization#tell-when-we're-in-a-server-context)