📝 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/response_model/tutorial004.py

@@ -1,4 +1,4 @@
-from typing import List, Optional
+from typing import List, Union
 
 from fastapi import FastAPI
 from pydantic import BaseModel
@@ -8,7 +8,7 @@ app = FastAPI()
 
 class Item(BaseModel):
     name: str
-    description: Optional[str] = None
+    description: Union[str, None] = None
     price: float
     tax: float = 10.5
     tags: List[str] = []