A very brief description (or motto) could be given when hovering on the flag.
But a more detailed description can be placed with the army page; accessible only to knights in the army, and/or under the list armies in the command section; accessible only to lords. I think I would suggest for it to be in both locations. For the lords to decide how they want their knights to serve, and for the knights to know the details of their army when they are in it.
