fileutils.rb: improve rdoc for FileUtils

	* lib/fileutils.rb: [DOC] fix invalid example code to make it
	  syntax highlighted, fix rdoc for lists, nodoc internal methods,
	  avoid a dangerous example.
	hash.c: [DOC] fix book title in example
	struct.c: improve rdoc for Struct

	* struct.c: [DOC] improve examples for ::new, improve #dig example,
	  simplify #select example, use consistent style for return values,
	  fix typos and example code style, remove duplicate cross reference.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_4@57918 b2dd03c8-39d4-4d8f-98ff-823fe69b080e