Core documentation about how the prelude is injected is wrong

[brson]

The docs for core include an explanation of what modules are injected by default, and that explanation is no longer correct. I believe the current situation is that core is imported at the top level of the crate and core::prelude::* is imported into every module.

[lifthrasiir]

It would be also good to indicate what modules are in the prelude and what are not. I didn't realized that core::repr is no longer imported automatically until turning RUST_LOG on.

[catamorphism]

This documentation should be updated for 0.6