Consider documenting the split extern prelude

[ehuss]

rust-lang/rust#144793 changed it so that the extern prelude is split in two. One for extern crate and one for --extern.

I'm wondering if this is something that should be documented. The existing extern prelude docs mention that it receives items from --extern and extern crate, but does not describe this new split behavior.

See also rust-lang/rust#145961