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]
merge revision(s) 57522: [Backport #13189]
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_3@58031 b2dd03c8-39d4-4d8f-98ff-823fe69b080e