Add comments on interaction between desturl and other dest* options

I've attached a patch for the duplicity action example configuration file as found in Debian package version 0.9.7-5. This patch is supposed to make it more clearly that you cannot combine a setting of 'desturl' with the other dest* options ('destuser', 'desthost', 'destdir'), 'bandwithlimit' or 'testconnect'.

Without this patch applied, the unaware user may run into warnings: Warning: testconnect can not be used when desturl is set Warning: the configured desthost is ignored since desturl is set Warning: the configured destdir is ignored since desturl is set Warning: The bandwidthlimit option is not used when desturl is set.

The comments added to the configuration file should save the user from unneccessarily running into these warnings.

(from redmine: created on 2010-04-14, closed on 2010-05-06)