Special case behavior of #![cfg(..)] at crate root needs documenting

[steveklabnik]

Originally at rust-lang/rust#34968

Spawned off of rust-lang/rust#34932 (comment)

If you put the inner attribute #![cfg(..)] at a crate root, this (apparently) causes an empty crate to be generated. I.e., it is as if the crate itself still exists, but all of its contents have disappeared.

This is slightly different than the behavior of #![cfg(..)] elsewhere (e.g. in a mod item), where such an inner attribute will cause the whole item to be omitted from the AST, rather than just causing the contents of the item to be omitted.

I don't see any mention of this corner case in the Rust documentation for Conditional Compilation, here: https://doc.rust-lang.org/book/conditional-compilation.html

[steveklabnik]

@QuietMisdreavus beat me to it #103