Add audit export to Briefcase documentation

Per https://github.com/opendatakit/briefcase/pull/641 we should document Briefcase's audit behavior and also add a reference to https://docs.opendatakit.org/form-audit-log/

Review Briefcase pull request #641 and the existing Sphinx documentation structure to identify where audit behavior is described. Add documentation for Briefcase's audit behavior and include a reference to https://docs.opendatakit.org/form-audit-log/; done means both topics are represented in the published documentation.

Written by the indexing model from the issue text.