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

[Shynur Xie]

> Stefan:
> Why do you care if it starts at 4 rather than at 5?

To be honest, I don't really care; it's quite easy for me to fix this
in my function (although I'm a little freaked out that the behavior
doesn't match the docstring: "accumulated number of garbage
collections done.").  Even if I just leave it there, it's only one
less than it actually is.

But at least I should let Emacs developers know this.  If this is not
an intended behavior, they can fix it; if it is, they may make it
clear in docstring.

So the question is:  do you care?  or, do you think users care?

If the answer is no, then there's no need to change it, just clarify
this behavior in the docstring of `post-gc-hook'.