Docs: Mention that G_VA_COPY() must be followed by `va_end()`

Hello!

I'm aware of at least one downstream project which doesn't do this and so is leaking the memory. See https://gitlab.xfce.org/xfce/exo/-/blob/master/exo/exo-job.c#L388

Maybe we should explicitly state that G_VA_COPY() must be matched with a corresponding va_end() call or there should be G_VA_END()?

Please feel free to close this MR if you think the suggested change is not required. :)

Thanks!