Add some docs

synapse/util/logcontext.py

@@ -134,13 +134,25 @@ class PreserveLoggingContext(object):
 
 
 def preserve_context_over_fn(fn, *args, **kwargs):
-    with PreserveLoggingContext():
-        deferred = fn(*args, **kwargs)
+    """Takes a function and invokes it with the given arguments, but removes
+    and restores the current logging context while doing so.
 
-    return preserve_context_over_deferred(deferred)
+    If the result is a deferred, call preserve_context_over_deferred before
+    returning it.
+    """
+    with PreserveLoggingContext():
+        res = fn(*args, **kwargs)
+
+    if isinstance(res, defer.Deferred):
+        return preserve_context_over_deferred(res)
+    else:
+        return res
 
 
 def preserve_context_over_deferred(deferred):
+    """Given a deferred wrap it such that any callbacks added later to it will
+    be invoked with the current context.
+    """
     d = defer.Deferred()
 
     current_context = LoggingContext.current_context()