g_object_get_property: document that value must be initialized

g_object_get_property: document that value must be initialized

I couldn't tell from reading the documentation whether I had to pass in
an uninitialized value, or a value initialized to the exact type, or
something else. It turns out (from reading the source) that you have to
pass in an initialized value, but you can use any type to which the
property's actual type can be transformed.

So, let's document this.