update comment

Signed-off-by: Binbin <[email protected]>

valkey.conf

@@ -1271,10 +1271,9 @@ acllog-max-len 128
 #    its primary, the content of the whole database is removed in order to
 #    load the RDB file just transferred.
 #
-# In all the above cases the default is to delete objects in a blocking way,
-# like if DEL was called. However you can configure each case specifically
-# in order to instead release memory in a non-blocking way like if UNLINK
-# was called, using the following configuration directives.
+# In all the above cases the old default is to delete objects in a blocking way,
+# like if DEL was called. Now the new default is release memory in a non-blocking
+# way like if UNLINK was called.
 
 lazyfree-lazy-eviction yes
 lazyfree-lazy-expire yes
@@ -1293,6 +1292,10 @@ lazyfree-lazy-user-del yes
 # commands. When neither flag is passed, this directive will be used to determine
 # if the data should be deleted asynchronously.
 
+# There are many problems with running flush synchronously. Even in single CPU
+# environments, the thread managers should balance between the freeing and
+# serving incoming requests. The default value is yes.
+
 lazyfree-lazy-user-flush yes
 
 ################################ THREADED I/O #################################