Update comments

src/Coordination/KeeperStateMachine.h

@@ -126,8 +126,11 @@ protected:
     /// Mutex for snapshots
     mutable std::mutex snapshots_lock;
 
+    /// Lock for the storage
+    /// Storage works in thread-safe way ONLY for preprocessing/processing
+    /// In any other case, unique storage lock needs to be taken
     mutable SharedMutex storage_mutex;
-    /// Lock for storage and responses_queue. It's important to process requests
+    /// Lock for processing and responses_queue. It's important to process requests
     /// and push them to the responses queue while holding this lock. Otherwise
     /// we can get strange cases when, for example client send read request with
     /// watch and after that receive watch response and only receive response

src/Coordination/KeeperStorage.cpp

@@ -772,6 +772,7 @@ void KeeperStorage<Container>::UncommittedState::cleanup(int64_t commit_zxid)
 template<typename Container>
 void KeeperStorage<Container>::UncommittedState::rollback(int64_t rollback_zxid)
 {
+    // we can only rollback the last zxid (if there is any)
     std::list<Delta> rollback_deltas;
     {
         std::lock_guard lock(deltas_mutex);