bug#24827: 24.5; Elisp manual: add index entries for syntax highlighting

[Drew Adams]

> > There is apparently no index entry that includes "highlight", and
> > in particular no index entries for "syntax highlighting",
> > "highlighting syntax", etc.
> 
> There is such an index entry in the Emacs user manual.

Great.  That's a good start.  Please consider adding it to
the Elisp manual as well.

Some (maybe most) users who want to implement or customize
syntax highlighting will go directly to the Elisp manual,
expecting (correctly) that this is done using Elisp.

And in fact one of the most relevant nodes for this topic
is `Customizing Keywords'.  "Customizing" is a human task -
this is a case where the Elisp manual (correctly) includes
some guide-like information.