bug#37787: [PATCH] Update `special-mode' docstring.

[Hong Xu]

On 10/17/19 8:37 PM, Lars Ingebrigtsen wrote:
> Hong Xu <hong@topbug.net> writes:
>
> > * lisp/simple.el (special-mode): Add more explanation for
> > `special-mode'.
>
> [...]
>
> > +A special major mode is intended to view specially formatted data
> > +rather than files.  It is read-only and usually lightweight."
>
> What do you mean by "lightweight"?  My guess is that, on average,
> special modes have more code than non-special modes.

Looks like I got that wrong. We can remove that part.