📝 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/dependencies/tutorial005.py

@@ -1,16 +1,17 @@
-from typing import Optional
+from typing import Union
 
 from fastapi import Cookie, Depends, FastAPI
 
 app = FastAPI()
 
 
-def query_extractor(q: Optional[str] = None):
+def query_extractor(q: Union[str, None] = None):
     return q
 
 
 def query_or_cookie_extractor(
-    q: str = Depends(query_extractor), last_query: Optional[str] = Cookie(default=None)
+    q: str = Depends(query_extractor),
+    last_query: Union[str, None] = Cookie(default=None),
 ):
     if not q:
         return last_query