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 (closed)