Add comment to clarify an intention

Signed-off-by: Yuta Iwama <[email protected]>

lib/fluent/plugin/out_forward.rb

@@ -619,6 +619,7 @@ def purge_obsolete_socks
           end
         end
 
+        # We expect that `yield` returns a unique object in this class
         def fetch_or(key = Thread.current.object_id)
           @mutex.synchronize do
             unless @active_socks[key]