Actually, I removed this line entirely here. It was part of the tip about resource gathering which ended up getting too long for the available space in the GUI. It had to be cut down and this line was not directly related to the topic anyway.
That's not to say this information shouldn't be included in the tips. In fact, I believe a separate tip about unit ranks would be quite useful. To my knowledge, players are currently not taught about the game's experience system anywhere. Definitely something to keep in mind for the future.