nit: update union

docs/concepts/union.md

@@ -1,7 +1,29 @@
-!!! warning "This page is a work in progress"
+Pydantic models also support `Union` types, which are used to represent a value that can be one of several types.
 
-You can use `Union` types to write *agents* that can dynamically choose actions - by choosing an output class.
-For example, in a search and lookup function, the LLM can determine whether to execute another search, lookup
-or other action, or finish the task with an answer.
+While many libraries support multiple function calls, and tool calls support multiple returns, the goal is to provide only one way to do things.
+
+## Unions for Multiple Types
+
+You can use `Union` types to write _agents_ that can dynamically choose actions - by choosing an output class. For example, in a search and lookup function, the LLM can determine whether to execute another search, lookup or other action.
+
+```python
+class Search(BaseModel):
+    query: str
+
+    def execute(self):
+        return ...
+
+class Lookup(BaseModel):
+    key: str
+
+    def execute(self):
+        return ...
+
+class Action(BaseModel):
+    action: Union[Search, Lookup]
+
+    def execute(self):
+        return self.action.execute()
+```
 
 See 'examples/union/run.py' for a working example.