bug#65700: time when gcs-done is updated needs to be clarified

[Eli Zaretskii]

> From: Shynur Xie <one.last.kiss@outlook.com>
> Date: Sat, 2 Sep 2023 13:00:59 +0000
> msip_labels: 
> 
> I wrote a function which runs each time Emacs collects garbage, so I
> added it to `post-gc-hook'.
> 
> That function cares about the value of `gcs-done' and the value of
> `gc-elapsed'.
> But it seems that `gcs-done' is updated after `post-gc-hook' is
> executed?
> 
> Is this an intended behavior?  If so, I think the docstring of these
> variables (such as `gcs-done', `gc-elapsed', ...) need to clarify when
> the variable is updated.

Stefan, any comments?