doc-export: add Fuse.Auth reference

[mortend]

Documentation for Fuse.Auth should be included in our API docs, and therefore also included in the doc-export project.

This PR contains:

• [ ] Changelog
• [ ] Documentation
• [ ] Tests

[mortend]

• [x] https://travis-ci.org/github/fuse-open/fuselibs/builds/751818268