To Kill a Mockingbird, doesn't mention the Ming dynasty because...well...it isn't relevant. Gee, no kidding. Well, that fact escapes many a fantasist who thinks that in order to create a believable world, you've gotta show all of your cards.
Which leads me to my second point: You can always tell when someone is lying, because they'll give you way too much information.
Why? The simple "truth" doesn't sound convincing to them because they know it's a lie. Therefore, they add detail to make it more believable.
Fantasy authors are without doubt the biggest bunch of liars around. We're not just lying about the story itself--the characters, the plot, the relationships--we're lying to you about EVERYTHING.
The grass. The trees. The water. Everything-freaking-thing.
So, how much is too much?
Good question. I think it's important for the author to know all of the ins and outs. But, pick only what's relevant to the story. Ask yourself if it were any other genre, would it be necessary. If not, scratch it.
In my humble opinion anyway.
OK, I kid, my opining is never humble. But, you get the gist here. This issue is one of the reasons that genre fiction gets the short end of the stick sometimes when compared to literary fiction. They know we're all full of crap.
We've just gotta become better liars...