Add comment for memory tracker blocking

src/Coordination/KeeperServer.cpp

@@ -176,6 +176,11 @@ struct KeeperServer::KeeperRaftServer : public nuraft::raft_server
 
     void commit_in_bg() override
     {
+        // For NuRaft, if any commit fails (uncaught exception) the whole server aborts as a safety
+        // This includes failed allocation which can produce an unknown state for the storage,
+        // making it impossible to handle correctly.
+        // We block the memory tracker for all the commit operations (including KeeperStateMachine::commit) 
+        // assuming that the allocations are small
         MemoryTrackerBlockerInThread blocker;
         nuraft::raft_server::commit_in_bg();
     }