bug#35702: xref revert-buffer

[Eli Zaretskii <eliz@gnu.org>]

> Cc: 35702@debbugs.gnu.org, juri@linkov.net
> From: Dmitry Gutov <dgutov@yandex.ru>
> Date: Sun, 26 May 2019 00:33:33 +0300
> 
> On 25.05.2019 19:49, Eli Zaretskii wrote:
> 
> > Then I already asked those questions.  The most important one is what
> > values can the fetcher variable have, and in what situations (i.e.,
> > triggered by what command(s)) will we see each value.
> 
> It's akin to asking what values could revert-buffer-function have: 
> different ones.

Although in theory there could indeed be an infinite number of values,
in practice the current actual set is very small, and can be easily
described.  If you want to avoid the (IMO imaginary) danger of
implying there could be no other values, you can say explicitly that
other values are possible.

IOW, useful documentation should be general, but not too general.  If
being general means refraining saying something that could potentially
help the reader understand the software and use it, then you are
probably on the wrong side of "too general".

> I have added some more docstrings and a comment, though. Hope they clear 
> up a few things.

Thanks.