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.