I don't think it should be made publicly available. It could spread to your realm's enemies, or to rivals within your realm?
And just how useful is it if it's also a secret to everyone in your own realm? Not useful at all is the answer.
I agree with Lilwolf, it makes no sense for SF special abilities to be completely opaque to the person leading them. Realistically, how are you *not* going to know what your unit can do? If they're more effective against fortifications, it's probably because they're lugging around cannons, bombs or other specialized siege equipment. Not exactly secret.
