bug#3506: 23.0.94; nil shouldn't be quoted in Info manuals

> Similarly, throughout the manual. Also, the Org manual uses 
> `nil' in at least some places.
> 
> (Personally, I think `nil' and `t' look better than nil and 
> t, but IIUC those two symbols are not supposed to be quoted.)

Apparently I was wrong about it being the convention that those two symbols not
be quoted. Looking at the manuals generally, they seem to be quoted. Dunno where
I got that idea; I thought I recalled Richard mentioning it as the convention to
follow.

So much the better.