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

[Winston]

Dmitry suggested:
> Maybe the manual should advertise the necessity to call etags with 
> --regexp in certain cases more prominently instead.

On the assumption that no one will read the manual until they have to,
I'd still suggest at least a 1-sentence mention of the issue in the
documentation for xref-find-definitions, even if it mostly says see the
manual if you're having problems finding tags.
 -WBE