document mail and mailAlternative attribute changes

The mail attribute is now synthesised based on the user's crsid and is released to external SPs. Document the change.

See https://gitlab.developers.cam.ac.uk/uis/devops/raven/ansible-shibboleth/-/issues/108 and https://gitlab.developers.cam.ac.uk/uis/devops/raven/ansible-shibboleth/-/merge_requests/82

Part of https://gitlab.developers.cam.ac.uk/uis/devops/raven/ansible-shibboleth/-/issues/120