Add reference to commit_message_model in git docs. (#33186)

Release Notes:

- N/A

docs/src/git.md

@@ -76,6 +76,20 @@ You can ask AI to generate a commit message by focusing on the message editor wi
 
 > Note that you need to have an LLM provider configured. Visit [the AI configuration page](./ai/configuration.md) to learn how to do so.
 
+You can specify your preferred model to use by providing a `commit_message_model` agent setting. See [Feature-specific models](./ai/configuration.md#feature-specific-models) for more information.
+
+```json
+{
+  "agent": {
+    "version": "2",
+    "commit_message_model": {
+      "provider": "anthropic",
+      "model": "claude-3-5-haiku"
+    }
+  }
+}
+```
+
 <!-- Add media -->
 
 More advanced AI integration with Git features may come in the future.