bug#22692: docstring for xref-find-definitions

[Juri Linkov]

>> It's a breaking one, but it primarily affects xref (aside from the
>> "obsolete" etags commands). And by doing it now, we can avoid similar
>> breakage in the next Emacs release.
>
> Exactly my thoughts.
>
> So I think you should push your change, unless Juri objects.

I'm fine with using bounds-of-thing-at-point if you think it's OK
to require thingatpt in core packages such as isearch.