looping_call() will wait for the given function to finish before sc…

…heduling another

Thanks to @erikjohnston for clarifying, #15743 (comment)

We don't have to worry about calls stacking up if the given function takes longer than the scheduled time.

synapse/util/__init__.py

@@ -116,6 +116,11 @@ def looping_call(
 
         Waits `msec` initially before calling `f` for the first time.
 
+        If the function given to `looping_call` returns an awaitable/deferred, the next call isn't
+        scheduled until after the returned awaitable has finished. We get this
+        functionality thanks to this function being a thin wrapper around
+        `twisted.internet.task.LoopingCall`.
+
         Note that the function will be called with no logcontext, so if it is anything
         other than trivial, you probably want to wrap it in run_as_background_process.