Now, that's true.
Here's a thought. Let's put things to the test with a real play example.
I'm playing in a Dungeon of the Mad Mage campaign. In the campaign we have discovered that some of the monsters (it appears to be mostly bugbears IIRC, or at least goblinoids), are infested with Intellect Devourers.
Now, I, the player, know that a Protection from Evil/Good will hedge out those monsters. I know that, but, it's very unlikely that my paladin would. That's a pretty esoteric piece of information. And, honestly, I asked the DM directly if my character would know that and he agreed with me that it was pretty unlikely, so, no.
So, now, in your game, what narration could I make to determine whether or not I knew that piece of information? How would you want your players to phrase things? In my group, I just stepped out of character, asked the DM directly and had a sidebar about it. But, I'm thinking that's not what you folks would want.
So, how can my paladin determine that Protection from Good/Evil is the way to hedge out Intellect Devourers.
((This is a much better example than the ubiquitous trolls and fire thing IMO, since trolls and fire is likely so commonly known - trolls aren't exactly rare. But, this is a far more esoteric bit of D&D trivia and I can't see it being common knowledge))