Ruby

set.rb: improve docs for Set

• lib/set.rb: [DOC] add examples for Set#replace,
  add examples for creating a set from a hash with duplicates,
  simplify and fix style of some other examples, fix typos.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@60879 b2dd03c8-39d4-4d8f-98ff-823fe69b080e