Fix DBUS query result parsing for secrets-search-items

Fix DBUS query result parsing for secrets-search-items

* lisp/net/secrets.el (secrets-search-items): Fix DBUS query result
parsing.  The function assumed that return value of the
SearchItems method called on a collection is a list of two lists,
however this is true only when no collection is specified.  GNOME
had used to incorrectly return a list of two lists in both cases,
but this was already fixed:
https://bugzilla.gnome.org/show_bug.cgi?id=695115 .  Also fix an
incorrect information in the secrets-search-items’ docstring.
(Bug#20449)

Copyright-paperwork-exempt: yes