updated to reflect r57515 (return value of each_pair).

Applied in changeset r57887.

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 ofmember'), which is clearer for users of the class
• improve code examples: e.g. use more consistent style (always use
  double quotes, drop p' andputs', ...), 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 ]

ruby_2_2 r58124 merged revision(s) 57887.

ruby_2_4 r61364 merged revision(s) 57887.

ruby_2_3 r62127 merged revision(s) 57887.