bug#27541: 25.2; Elisp manual: Please add index entries for "rectangle"

[Eli Zaretskii]

> Date: Fri, 30 Jun 2017 13:57:40 -0700 (PDT)
> From: Drew Adams <drew.adams@oracle.com>
> 
> Subject line says it all.  There are plenty of places in the Elisp
> manual where various kinds of rectangle thingie are presented.  Please
> add some index entries for such things.

I found only one: in "Regsiters", so I added an index entry there.

Thanks.