If you let some players decide, they grab a MM and start reading the specs saying "But my character knows all this from being a librarian", ruining it for all other players...
Almost all players know what to do with trolls, but unless they are common to the area the character is from, lore about them isn't readily known IMHO.
Naturally, players will discover soon enough how to really damage them and after the first hit start to ask what can be done about the regen...
BUT: The next time they encounter a troll, they know immediately what to do!