bug#28403: 25.2; find-tag works, but xref-find-definitions

[Winston]

[Eli also pointed out that TAB is the way to see possible completions
 for a tag string.  It worked.  Thanks to both of you.]

I asked:
>> Is there an "always prompt" option, or do I need to write a trivial
>> wrapper function?

Dmitry kindly replied:
> You can customize xref-prompt-for-identifier to t.

Hooray!  Thanks!
 -WBE