Describe "Deprecated APIs removed." and "struct RBignum is hidden."


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44963 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
Add note about "struct RBignum is hidden.".


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44964 b2dd03c8-39d4-4d8f-98ff-823fe69b080e