Mention Learn ahead limit in FAQ

I'm surprised we don't already do this. There seems to be a lot of people who don't understand Learn ahead and get confused when their red/again/learning cards don't appear quickly (remember somebody once saying as /j that's the question half of all redditors ask). We should have a new page addressing this. @Danika-Dakika Thoughts? How should the question(s) in FAQ be framed?

Start by locating the FAQ and existing Learn ahead documentation, then review how the limit and card states are currently explained. Clarify the question framing before adding the new page or FAQ entry; done means users can understand why red, again, and learning cards may not appear immediately.

Written by the indexing model from the issue text.