Extend D-Bus doc and test

* doc/misc/dbus.texi (Register Objects): Adapt doc of
dbus-unregister-service.

* test/lisp/net/dbus-tests.el (dbus--test-register-service):
Extend test.

doc/misc/dbus.texi

@@ -1418,6 +1418,9 @@ The name @var{service} does not exist on the bus.
 @item :not-owner
 We are not an owner of the name @var{service}.
 @end table
+
+When @var{service} is not a known name but a unique name, the function
+returns nil.
 @end defun
 
 When a name has been chosen, Emacs can offer its own methods, which

test/lisp/net/dbus-tests.el

@@ -465,6 +465,9 @@
   (should (eq (dbus-unregister-service bus dbus--test-service) :non-existent))
   (should-not (member dbus--test-service (dbus-list-known-names bus)))
 
+  ;; Unregistering a unique name returns nil.
+  (should-not (dbus-unregister-service bus ":1.1"))
+
   ;; A service name is a string, constructed of at least two words
   ;; separated by ".".
   (should