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

[Stefan Monnier]

>> 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.
>
>> Eli:
>> That's just causes off-by-one count of GC cycles, no?  Easy enough
>> to fix.
>
>> Stefan:
>> the only difference is that `gcs-done` will be always smaller by 1
>
> We forgot the `gc-elapsed'.  It's a little hard to fix (dirty work is
> needed).

Yup, for `gcs-done`, it doesn't matter very much, but with the current
code `post-gc-hook` simply can't know how much time the last GC took,
which can be a useful information there.
I'll push my patch to `master` soon, I just need to clean it up first.


        Stefan