* lib/pp.rb, lib/prettyprint.rb: Documentation for PP and PrettyPrint
  Based on a patch by Vincent Batts [ruby-core:51253] [Bug #7656]


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39180 b2dd03c8-39d4-4d8f-98ff-823fe69b080e