add some note about no more write requests should be sent to TiKV dur…

…ing the truncate process

Signed-off-by: longfangsong <longfangsong@icloud.com>

text/0081-truncate.md

@@ -40,7 +40,7 @@ service Tikv {
 
 When a TiKV receive `TruncateRequest`, it will create a `TruncateWorker` instance and start the work.
 
-There are several steps to follow when doing truncate:
+There are several steps to follow when doing truncate, note it is expected that no more write requests will be sent to TiKV during the truncate process:
 
 ### 1. Wait all committed raft log are applied