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

diff --cc src/doc/reference.md
Simple merge

diff --cc src/librustc/middle/ty.rs
Simple merge

diff --cc src/libsyntax/ast.rs
Simple merge