📝 Add docs recommending Union over Optional and migrate source examples (#4908)

* 📝 Add docs recommending Union over Optional

* 📝 Update docs recommending Union over Optional

* 📝 Update source examples for docs, recommend Union over Optional

* 📝 Update highlighted lines with updated source examples

* 📝 Update highlighted lines in Markdown with recent code changes

* 📝 Update docs, use Union instead of Optional

* ♻️ Update source examples to recommend Union over Optional

* 🎨 Update highlighted code in Markdown after moving from Optional to Union

docs_src/schema_extra_example/tutorial004.py

@@ -1,4 +1,4 @@
-from typing import Optional
+from typing import Union
 
 from fastapi import Body, FastAPI
 from pydantic import BaseModel
@@ -8,9 +8,9 @@ app = FastAPI()
 
 class Item(BaseModel):
     name: str
-    description: Optional[str] = None
+    description: Union[str, None] = None
     price: float
-    tax: Optional[float] = None
+    tax: Union[float, None] = None
 
 
 @app.put("/items/{item_id}")