📝 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/query_params/tutorial004.py

@@ -1,4 +1,4 @@
-from typing import Optional
+from typing import Union
 
 from fastapi import FastAPI
 
@@ -7,7 +7,7 @@ app = FastAPI()
 
 @app.get("/users/{user_id}/items/{item_id}")
 async def read_user_item(
-    user_id: int, item_id: str, q: Optional[str] = None, short: bool = False
+    user_id: int, item_id: str, q: Union[str, None] = None, short: bool = False
 ):
     item = {"item_id": item_id, "owner_id": user_id}
     if q: