rollup merge of #22094: alkor/cleanup-show-string

Rename several remaining `Show`s to Debug, `String`s to Display (mostly in comments and docs).
Update reference.md:
 - derive() no longer supports Zero trait
 - derive() now supports Copy trait

src/libsyntax/ast.rs

@@ -64,7 +64,6 @@ use parse::token;
 use ptr::P;
 
 use std::fmt;
-use std::fmt::Show;
 use std::num::Int;
 use std::rc::Rc;
 use serialize::{Encodable, Decodable, Encoder, Decoder};