fix(incremental sync): added comment for 2 times do_sync (#13801)

Co-authored-by: Datong Sun <datong.sun@konghq.com>

kong/clustering/services/sync/rpc.lua

@@ -334,7 +334,9 @@ local function sync_handler(premature)
   end
 
   local res, err = concurrency.with_worker_mutex(SYNC_MUTEX_OPTS, function()
-    -- here must be 2 times
+    -- `do_sync()` is run twice in a row to report back new version number
+    -- to CP quickly after sync. (`kong.sync.v2.get_delta` is used for both pulling delta
+    -- as well as status reporting)
     for _ = 1, 2 do
       local ok, err = do_sync()
       if not ok then