doc: restore class documentation for Struct

* struct.c: restore class documentation for Struct
  that disappeared with r46663.

Due to r46663, the class documentation for Struct disappeared.
(The revision inserted the definition of `InitVM_Struct` between
the rdoc and the definition of `Init_Struct`.)

The docs are rendered for 2.1: <https://docs.ruby-lang.org/en/2.1.0/Struct.html>,
but not for later versions, see: <https://docs.ruby-lang.org/en/2.2.0/Struct.html>
(Same for `ri` pages).

[ruby-core:79416] [Bug #13189]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@57522 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

struct.c

@@ -1158,6 +1158,8 @@ rb_struct_dig(int argc, VALUE *argv, VALUE self)
 }
 
 /*
+ *  Document-class: Struct
+ *
  *  A Struct is a convenient way to bundle a number of attributes together,
  *  using accessor methods, without having to write an explicit class.
  *