string.c: restore documentation for String#<<
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@57758 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
string.c: restore documentation for String#<<
which became undocumented with r56021; fix a typo.
[ruby-core:79865] [Bug #13268]
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@57758 b2dd03c8-39d4-4d8f-98ff-823fe69b080e