The description says:

Dear Princess Celestia,

Today I learned a very important lesson, not about friendship but of reading. I have learned to ALWAYS read the fine print. I am also guessing that Star Swirl thought it was funny to turn ponies into Alicorns, causing them to gain untold amounts of stress and paperwork to do, just for writing in his book.

Your faithful student, Princess Twilight Sparkle.

Princess Luna must have wanted me to tell the world the true reason of Twilight’s evolution because this idea came to me in a dream believe it or not.