add some comments

Author: Xiao Li

src/diem/jsonrpc/async_client.py

@@ -159,6 +159,9 @@ async def __aenter__(self) -> "AsyncClient":
         return self
 
     async def __aexit__(self, *args, **kwargs) -> None:  # pyre-ignore
+        """use async with to ensure close the client session after used"""
+
+        # we don't use `__del__` because close session requires `await`.
         await self.close()
 
     # high level functions

src/diem/testing/miniwallet/app/app.py

@@ -138,6 +138,8 @@ async def worker() -> None:
                 try:
                     await asyncio.gather(*[t() for t in self.bg_tasks])
                 except asyncio.CancelledError:
+                    # when application is shutting down, tasks maybe cancelled.
+                    # ignore the error and shutdown the worker.
                     return
                 except Exception as e:
                     self.logger.exception(e)