docs: how to add function calling to custom engine

docs/engines.rst

@@ -44,6 +44,34 @@ the underlying model, and kani needs to know about the extra tokens added by thi
   model.
 - :meth:`.BaseEngine.close`: if your engine needs to clean up resources during shutdown.
 
+Adding Function Calling
+^^^^^^^^^^^^^^^^^^^^^^^
+If you're writing an engine for a model with function calling, there are a couple additional steps you need to take.
+
+Generally, to use function calling, you need to do the following:
+
+1. Tell the model what functions it has available to it
+    a. Optional - tell the model what format to output to request calling a function (if the model is not already
+       fine-tuned to do so)
+2. Parse the model's requests to call functions from its text generations
+
+To tell the model what functions it has available, you'll need to somehow prompt the model.
+You'll need to implement two methods: :meth:`.BaseEngine.predict` and :meth:`.BaseEngine.function_token_reserve`.
+
+:meth:`.BaseEngine.predict` takes in a list of available :class:`.AIFunction`\ s as an argument, which you should use to
+build such a prompt. :meth:`.BaseEngine.function_token_reserve` tells kani how many tokens that prompt takes, so the
+context window management can ensure it never sends too many tokens.
+
+To parse the model's requests to call a function, you also do this in :meth:`.BaseEngine.predict`. After generating the
+model's completion (usually a string, or a list of token IDs that decodes into a string), separate the model's
+conversational content from the structured function call:
+
+.. image:: _static/function-calling-parsing.png
+    :align: center
+
+Finally, return a :class:`.Completion` with the ``.message`` attribute set to a :class:`.ChatMessage` with the
+appropriate :attr:`.ChatMessage.content` and :attr:`.ChatMessage.function_call`.
+
 HTTP Client
 -----------
 If your language model backend exposes an HTTP API, you can create a subclass of :class:`.BaseClient` to interface with

docs/kani.rst

@@ -15,6 +15,12 @@ Let's take a look back at the quickstart program:
 kani is comprised of two main parts: the *engine*, which is the interface between kani and the language model,
 and the *kani*, which is responsible for tracking chat history, prompting the engine, and handling function calls.
 
+.. image:: _static/concepts-figure.png
+    :width: 60%
+    :align: center
+
+In this section, we'll look at how to initialize a Kani class and core concepts in the library.
+
 Kani
 ----
 
@@ -100,8 +106,8 @@ This table lists the engines built in to kani:
 
 When you are finished with an engine, release its resources with :meth:`.BaseEngine.close`.
 
-Chat Messages
--------------
+Concept: Chat Messages
+----------------------
 Each message contains the ``role`` (a :class:`.ChatRole`: system, assistant, user, or function) that sent the message
 and the ``content`` of the message. Optionally, a user message can also contain a ``name`` (for multi-user
 conversations), and an assistant message can contain a ``function_call`` (discussed in :doc:`function_calling`).