Document looping_call() functionality that will wait for the given …

…function to finish before scheduling another (#15772)

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.

changelog.d/15772.doc

@@ -0,0 +1 @@
+Document `looping_call()` functionality that will wait for the given function to finish before scheduling another.

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.