Update wrap_classes.rst with newest changes in autowrap

[jpfeuffer]

I think parts of it are not up-to-date anymore.
E.g. I think you should use const specifiers now. Especially on libcpp objects like lists, where you can skip the copy-back step if const.
For normal wrapped classes it is just cleaner if the (advanced) user looks at the Cython signature.

[timosachsenberg]

todo add how to wrap static functions