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

[Shynur Xie]

> Stefan:
> I'd be curious to know how it affects your code.

My Emacs displays its runtime information like this:
  <github.com/shynur/.emacs.d/issues/1#issuecomment-1671815270>
Since it's mainly related to GC, I have it updated after each GC.

I believe I'm not the first person to do this; why others didn't
report the bug may be that they don't care whether the number of GCs
is exactly the same as they expect or one less, so they didn't really
check when `gcs-done' is updated.
IOW, if the update time of `gcs-done' is changed, I think most people
won't notice this change at all.

> Stefan:
> "Philosophically", both choices make sense (either consider
> `post-gc-hook` as being part of the GC or consider it as external to
> the GC).

I'm not a native English speaker.  Does the POST in `post-gc-hook'
mean the functions will run when GC is DONE?

> shynur:
> 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.

And `garbage-collection-messages'.