lib/ostruct.rb: [DOC] revise docs for OpenStruct

	* update paragraph on implementation:
	  define_singleton_method is used, not define_method
	* add call-seq with return values for each_pair
	* adopt description of dig from Array and Hash
	* fix description of the hash method
	* :nodoc: initialize_copy, respond_to_missing?
	* other small improvements, e.g. use the term `attribute' in the docs
	  (instead of `member'), which is clearer for users of the class
	* improve code examples: e.g. use more consistent style (always use
	  double quotes, drop `p' and `puts', ...), update inspect output,
	  use example data that is not prone to change (like population)
	* add more code examples
	* fix some small errors and grammar

	[ruby-core:79265] [Bug #13159]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_4@61364 b2dd03c8-39d4-4d8f-98ff-823fe69b080e