Added a deprecation warning to the Custom Functions section of the documentation

Author: Valentin Foucault

fern/assistants/function-calling.mdx

@@ -4,7 +4,6 @@ subtitle: 'Additional Capabilities for Your Assistants '
 slug: assistants/function-calling
 ---
 
-
 Vapi voice assistants are given three additional functions: `transferCall`,`endCall`, and `dialKeypad`. These functions can be used to transfer calls, hang up calls, and enter digits on the keypad.
 
 <Note>You **do not** need to add these functions to your model's `functions` array.</Note>
@@ -71,6 +70,8 @@ This function is provided when `dialKeypadFunctionEnabled` is enabled on the ass
 
 ### Custom Functions
 
+<Warning>The **Custom Functions** feature is being deprecated in favor of [Tools](/tools-calling). Please refer to the **Tools** section instead. We're working on a solution to migrate your existing functions over to make this a seamless transtion.</Warning>
+
 In addition to the predefined functions, you can also define custom functions. These functions are similar to OpenAI functions and your chosen LLM will trigger them as needed based on your instructions.
 
 The functions array in the assistant definition allows you to define custom functions that the assistant can call during a conversation. Each function is an object with the following properties: