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

docs/src/app/docs/customization/page.mdx

@@ -62,6 +62,8 @@ export const env = createEnv({
 });
 ```
 
+If you want to reuse above `isServer` check in multiple places, you can use the [`shared` section](./recipes#isserver).
+
 ## Treat empty strings as undefined
 
 By default, T3 Env will feed the environment variables directly to the Zod validator. This means that if you have an empty string for a value that is supposed to be a number (e.g. `PORT=` in a ".env" file), Zod will flag

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

@@ -82,3 +82,22 @@ const config: StorybookConfig = {
 
 export default config;
 ```
+
+## isServer
+
+Determining whether the code is running on the server or client is a common requirement.
+You can use the `shared` section to define an environment variable that's accessible in both contexts:
+
+```ts
+export const env = createEnv({
+  shared: {
+    IS_SERVER: z.boolean().default(false),
+  },
+  runtimeEnv: {
+    IS_SERVER: typeof window === "undefined",
+  }
+})
+```
+
+Make sure to read the [customization](./customization#tell-when-we're-in-a-server-context) docs to learn how to override the `isServer` value used by the library itself.
+