bug#11178: 24.0.94; Elisp manual: add index entries for :link LINK-DATA alternatives

[Drew Adams]

> > These are reference entries.  No different from indexing 
> > individual function names or individual button properties.
> 
> Perhaps adding two more index entries, something like
> 
>   @cindex links to documentation for customization items
>   @cindex customization items, links to documentation
> 
> would do the job.  Currently, I see only one index entry there:
> 
>   @kindex link@r{, customization keyword}
> 
> which IMO is not enough for when the reader wants to find information
> about adding links to the docs.  The existing index entry only covers
> efficiently the case when the reader is specifically looking for the
> 'link' keyword and already knows what that's used for.

What Eli says is also pertinent.  But independent.

Again, you are talking about indexing _topics_.  I am talking about _reference_
index entries.

If a user wants to look up the particular button property `follow-link', she
does `i follow-link' and Bob's an uncle.

This is about looking up a particular :link alternative, such as
`emacs-commentary-link'.  It is not about looking up the topic of "links to
documentation for customization items".