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

[Sebastian Urban]

> But I see nothing wrong with the doc string as it is.

Right now the string is:
  Set one end of the secondary selection to the position clicked on.
  Use <M-mouse-3> to set the other end
  and complete the secondary selection.

As you can see - no information about deselect, so perhaps just
copy-paste last sentence from your patch - "This command cancels any
existing secondary selection, when it starts a new one."

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

Alright, but I think:
   use @kbd{M-mouse-3} to set the other end and complete the selection.

that this sentence from the patch is unnecessary.  How to set
secondary selection is in description of 'M-mouse-3' which is just ONE
line below.  In doc string of 'mouse-start-secondary' it is OK,
because there is no description of 'M-mouse-3', but in the manual
there is.