Update E0446.md
The existing error documentation did not show how to use a child module's functions if the types used in those functions are private. These are some other places this problem has popped up that did not present a solution (these are from before the solution existed, 2016-2017. The solution was released in the Rust 2018 edition. However these were the places I was pointed to when I encountered the problem myself):
https://github.com/rust-lang/rust/issues/30905
https://stackoverflow.com/questions/
39334430/how-to-reference-private-types-from-public-functions-in-private-modules/
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