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

[Eli Zaretskii <eliz@gnu.org>]

> From: "Drew Adams" <drew.adams@oracle.com>
> Date: Tue, 10 Apr 2012 23:27:43 -0700
> Cc: 11178@debbugs.gnu.org
> 
> > > (elisp) Common Keywords should be indexed for each of the LINK-DATA
> > > alternatives: `custom-manual', `info-link', `url-link',
> > > `emacs-commentary-link', `emacs-library-link', `file-link',
> > > `function-link', `variable-link', and `custom-group-link'.
> > 
> > No.  That makes no sense---there's already an index entry for adding
> > links to custom docs.  I don't want to pollute the index with entries
> > listing every possible value for what's just a keyword.  (Though I
> > eagerly await the coming long-form article arguing against this.)
> 
> 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.