📝 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/background_tasks/tutorial002.py

@@ -1,4 +1,4 @@
-from typing import Optional
+from typing import Union
 
 from fastapi import BackgroundTasks, Depends, FastAPI
 
@@ -10,7 +10,7 @@ def write_log(message: str):
         log.write(message)
 
 
-def get_query(background_tasks: BackgroundTasks, q: Optional[str] = None):
+def get_query(background_tasks: BackgroundTasks, q: Union[str, None] = None):
     if q:
         message = f"found query: {q}\n"
         background_tasks.add_task(write_log, message)