Reference: Fix list in Expressions section
auto merge of #18924 : cakebaker/rust/fix_list, r=steveklabnik
Try to remove ty_nil, some kind of error in exhaustiveness checking
Complete the removal of ty_nil, ast::LitNil, ast::TyBot and ast::TyUniq

[breaking-change]

This will break any uses of macros that assumed () being a valid literal.
Update tests accordingly
Update the reference