Re: master b02c9bc: Improve documentation of new Xref options

[Eli Zaretskii <eliz@gnu.org>]

> Cc: emacs-devel@gnu.org
> From: Dmitry Gutov <dgutov@yandex.ru>
> Date: Wed, 8 Sep 2021 03:24:12 +0300
> 
> On 07.09.2021 19:23, Eli Zaretskii wrote:
> >>> No, it's about the features listed after the shown hunk.  I just made
> >>> its language less extreme, because xref-find-apropos does exist.
> >> tags-apropos-additional-actions, then? That seems easier to support.
> > Yes, pretty much.
> 
> Here's a quick implementation.

Thanks.

> But the feature doesn't seem to be particularly popular/polished: the 
> example value for Elisp, for instance, sets up search across all symbols 
> (obarray), but subsequent navigation only works for commands, and only 
> ones documented in the manual.

I don't think we should be too bothered about that: once we have
replacements for all of those etags features, we could declare the
originals obsolete and point to replacements, something we cannot do
when the replacements are missing.  So I think we should install this.

>  (cl-defmethod xref-backend-apropos ((_backend (eql 'etags)) pattern)
> -  (etags--xref-find-definitions (xref-apropos-regexp pattern) t))
> +  (let ((regexp (xref-apropos-regexp pattern)))
> +    (nconc
> +     (etags--xref-find-definitions regexp t)
> +     (etags--xref-apropos-additional regexp))))

I'm not sure I understand why is this specific to the etags backend.
The spec seems to be more general, and xref-find-apropos is not
specific to etags, right?

We'd also need a defcustom, similar to tags-apropos-additional-actions.