Document D-Bus monitoring

Document D-Bus monitoring

* doc/misc/dbus.texi: Replace "symbol" by "keyword" where appropriate.
(Alternative Buses): Adapt dbus-init-bus description.
(Errors and Events): Adapt dbus-event structure.  New defuns
dbus-event-destination-name, dbus-event-handler and dbus-event-arguments.
(Monitoring Events): New node.

* lisp/net/dbus.el: Replace "symbol" by "keyword" where appropriate.
(cl-lib): Require.
(dbus-register-monitor): Adapt the argument list.
(dbus-monitor-handler): Extend.
(dbus-init-bus): Adapt docstring.

* test/lisp/net/dbus-tests.el (dbus-test01-compound-types):
Skip if needed.  Extend test.