marshal.c: warn against using Marshal.load on untrusted data

* marshal.c (marshal_load): Add documentation warning against using
  Marshal.load on untrusted data [Bug #7759] [ruby-core:51765]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@38999 b2dd03c8-39d4-4d8f-98ff-823fe69b080e