docs: fix a misunderstanding in g_type_add_interface_*
docs: fix a misunderstanding in g_type_add_interface_*

The previous documentation said this:
  g_type_add_interface_static:
    "Adds the static interface_type to instantiable_type"
  g_type_add_interface_dynamic:
    "Adds the dynamic interface_type to instantiable_type"

The above suggests that if one is adding a static interface to a dynamic
object, one should use g_type_add_interface_static because the interface
is static, but the code and usage (with the newly added
G_IMPLEMENTS_INTERFACE_DYNAMIC) imply that this is wrong, and that
what matters is whether the *instanciable_type* is dynamic or not.

Hence this patch moves the "static" and "dynamic" words close to
"instantiable_type".

Closes issue #259

endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib