bug#36365: Add info how to deselect secondary selection (Emacs manual)

[Drew Adams]

> I see now that there's no way of canceling a secondary selection
> without starting a new one.  So I changed the text to reflect that.

Correct.  An empty secondary selection is the same as
no secondary selection (at least as far as Emacs is
concerned).  When you try to yank the empty secondary
selection Emacs raises the error "No secondary selection".

(That's as it should be, IMO.)