Update the sample config

docs/sample_log_config.yaml

@@ -28,14 +28,17 @@ handlers:
     # will be a delay for INFO/DEBUG logs to get written, but WARNING/ERROR
     # logs will still be flushed immediately.
     buffer:
-        class: logging.handlers.MemoryHandler
+        class: synapse.logging.handlers.PeriodicallyFlushingMemoryHandler
         target: file
         # The capacity is the number of log lines that are buffered before
         # being written to disk. Increasing this will lead to better
         # performance, at the expensive of it taking longer for log lines to
         # be written to disk.
         capacity: 10
         flushLevel: 30  # Flush for WARNING logs as well
+        # The period of time, in seconds, between forced flushes.
+        # Messages will not be delayed for longer than this time.
+        period: 5
 
     # A handler that writes logs to stderr. Unused by default, but can be used
     # instead of "buffer" and "file" in the logger handlers.