Fix shortdoc-add-function section creation

Fix shortdoc-add-function section creation

* lisp/emacs-lisp/shortdoc.el (shortdoc-add-function): Use nconc to
actually append a new section to the list of groups while avoiding a
previous OBOE.  Push a new group to the front of shortdoc--groups
without copying it, just like define-short-documentation-group does.
(buffer): Fix copypasta in unlock-buffer example.

* test/lisp/emacs-lisp/shortdoc-tests.el (shortdoc-examples): Also
check that :no-value forms demonstrate the right function.

* doc/lispref/help.texi (Documentation Groups): Clarify that @dots
in the define-short-documentation-group arglist refer to whole
key-value pairs.  Fix typo in :eg-result-string description.