Document anyio requirement for trio support

Add clear documentation about anyio being required for proper trio support,
and the fallback to asyncio.Lock when anyio is not available.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: proboscis

returns/primitives/reawaitable.py

@@ -157,4 +157,4 @@ def decorator(
     ) -> _AwaitableT:
         return ReAwaitable(coro(*args, **kwargs))  # type: ignore[return-value]
 
-    return decorator
+    return decorator