[DOC] `IO.new` accepts `external_encoding`

Revert part of r61278 [Bug #13655]
[ci skip]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@61281 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

io.c

@@ -8043,6 +8043,9 @@ rb_io_make_open_file(VALUE obj)
  *    Specifies file open flags as integer.
  *    If +mode+ parameter is given, this parameter will be bitwise-ORed.
  *
+ *  :\external_encoding ::
+ *    External encoding for the IO.
+ *
  *  :\internal_encoding ::
  *    Internal encoding for the IO.  "-" is a synonym for the default internal
  *    encoding.