Add some documentation to AwaitRemove

src/textual/await_remove.py

@@ -1,13 +1,23 @@
+"""Provides the type of an awaitable remove."""
+
 from asyncio import Event
 from typing import Generator
 
 
 class AwaitRemove:
+    """An awaitable returned by App.remove and DOMQuery.remove."""
+
     def __init__(self, finished_flag: Event) -> None:
+        """Initialise the instance of ``AwaitRemove``.
+
+        Args:
+            finished_flag (asyncio.Event): The asyncio event to wait on.
+        """
         self.finished_flag = finished_flag
 
     def __await__(self) -> Generator[None, None, None]:
         async def await_prune() -> None:
+            """Wait for the prune operation to finish."""
             await self.finished_flag.wait()
 
         return await_prune().__await__()