No game provides tool tips for every aspect of everything in the game. This is why the Internet was invented, so that we could have wikis to give us all the info.
https://bg3.wiki/wiki/RageI am very much against expecting players to learn mechanics from sources outside of a game, but even to be generous, this isn't some obscure exception or unintuitive combination. Rage is the classes central feature. Why on earth would that not be fully explained? If you're playing a barbarian it's the one thing you most need to understand, not explaining it fully is just a failure on the part of the game.
