As @Don_Biglia mentioned, there's no way of knowing what a manager writes on his notepad. But some intelligent guesswork would suggest he/she is keeping a note of certain points about the game. It could be anything from a list of things to say at half time to individual advice to be given to certain players. Of course for all we know he/she could just be doodling as well, probably a technique to keep oneself calm. Many managers do not write anything, leaving that job to one of his/her assistants. Basically, it's up to the individual manager to do what he/she wants during a match, as long as he/she stays within the technical area and does not engage in any unsportsmanlike behaviour.