Add Function guide (#53424)

GitOrigin-RevId: 629812c750c98485c3ec5ae114d77992395f69b3

Author: mwaskom

go/modal_docs/intro.md

@@ -142,7 +142,7 @@ if !ok {
 fmt.Println(response)
 ```
 
-All Function invocation methods are available:
+Synchronous and asynchronous [invocation methods](/docs/guide/function-invocation-methods) are both supported:
 
 ```go
 fc, err := f.Spawn(ctx, []any{"Hello, Modal!"}, nil)
@@ -157,6 +157,8 @@ if err != nil {
 fmt.Println(result)
 ```
 
+Batched invocation is not currently supported from the Go SDK.
+
 ## Errors
 
 Error types are documented on the [Errors](/docs/sdk/go/latest/Errors) page. Errors are returned as values and implement the standard `error` interface; use `errors.As` to match them.

js/modal_docs/intro.md

@@ -108,14 +108,16 @@ ret = await echo.remote([], { s: "Hello, Modal!" });
 console.log(ret);
 ```
 
-All Function invocation methods are available:
+Synchronous and asynchronous [invocation methods](/docs/guide/function-invocation-methods) are both supported:
 
 ```typescript
 const functionCall = await echo.spawn(["Hello, Modal!"]);
 const ret = await functionCall.get();
 console.log(ret);
 ```
 
+Batched invocation is not currently supported from the JS SDK.
+
 ## Errors
 
 Error types are documented on the [Errors](/docs/sdk/js/latest/Errors) page. Methods throw typed error classes; use `instanceof` to match them.