- Make the rules for including headers explicit
- Make the symbol visibility rules explicit, and drop the
old "leading underscore" hack
- Drop Private data structure declarations and priv pointers
from public headers
- Mention G_DECLARE_* macros
- Mention `#pragma once`
We should mention that newly written code should not have the private
data pointer in the instance structure, and that private data should be
added using the new GObject macros.
https://bugzilla.gnome.org/show_bug.cgi?id=702996
