/assignee command explanation has to be smart(with Multiple Assignees feature)
When we are on issue page we have to see autocomplete tip like /assignee @user1[, @user] when we are on MR page it's only one assignee is possible so we have to see tip like /assignee @user
The explanation should be also different for issue and MR. The term "explanation" is taken from the code, it's responsible for preview tab mostly.