docs: Make *_INSIDE_* macros private

docs/reference/garcon-docs.xml

docs/reference/garcon-sections.txt

garcon-gtk/garcon-gtk-menu.h

garcon-gtk/garcon-gtk.h

garcon/garcon-config.h.in

garcon/garcon-environment.h

garcon/garcon-menu-directory.h

garcon/garcon-menu-element.h

garcon/garcon-menu-item-action.h

garcon/garcon-menu-item-cache.h

garcon/garcon-menu-item-pool.h

garcon/garcon-menu-item.h

garcon/garcon-menu-merger.h

garcon/garcon-menu-node.h

garcon/garcon-menu-parser.h

garcon/garcon-menu-separator.h

garcon/garcon-menu-tree-provider.h

garcon/garcon-menu.h

garcon/garcon-private.h

garcon/garcon.h