fasagreement: add doc string with explanation and examples

[tiran]

PR https://github.com/fedora-infra/freeipa-fas/pull/105 adds fasagreement but has no examples yet. IPA's plugin system uses the doc string for ipa help, e.g. https://github.com/freeipa/freeipa/blob/master/ipaserver/plugins/group.py#L62