io.c: fix rdoc of append mode
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46876 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
io.c: fix rdoc of append mode
not move the pointer at open. [ruby-core:63747] [Bug #10039]
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46876 b2dd03c8-39d4-4d8f-98ff-823fe69b080e