[Cherry-Pick]Fix reducer warning & fix doc of fleet (#29333)

* fix the warning of reducer (#29323)

* fix warning of fleet (#29317)

* Fix doc of fleet api (#29282)

paddle/fluid/imperative/reducer.cc

@@ -225,7 +225,7 @@ void Reducer::MarkVariableReady(const VariableIndex &var_index,
 
 void Reducer::MarkGroupReady(size_t group_index) {
   if (group_index > next_group_) {
-    LOG(WARNING) << "Maybe it need adjust the order of group";
+    VLOG(3) << "Maybe it need adjust the order of group";
     return;
   }
 

python/paddle/distributed/fleet/base/fleet_base.py

@@ -615,10 +615,10 @@ class Fleet(object):
 
         if strategy is not None:
             warnings.warn(
-                "It is recommended to pass in DistributedStrategy"
-                "in fleet.init. The strategy here is for compatibility."
-                "If the `strategy` in fleet.distributed_optimizer() is"
-                "not None, then it will overwrite the DistributedStrategy in fleet.init(),"
+                "It is recommended to use DistributedStrategy "
+                "in fleet.init(). The strategy here is only for compatibility. "
+                "If the strategy in fleet.distributed_optimizer() is "
+                "not None, then it will overwrite the DistributedStrategy in fleet.init(), "
                 "which will take effect in distributed training.")
             self._user_defined_strategy = copy.deepcopy(strategy)