Make sure __doc__ is cloned when a plugin is inherited in PluginContainer.
Make sure __doc__ is cloned when a plugin is inherited in PluginContainer.

pombredanne/https-git.fedorahosted.org-git-kobo,pombredanne/https-git.fedorahosted.org-git-kobo,pombredanne/https-git.fedorahosted.org-git-kobo,release-engineering/kobo,release-engineering/kobo,pombredanne/https-git.fedorahosted.org-git-kobo,release-engineering/kobo,release-engineering/kobo