The current configuration for the OIDCRedirectURI results in
mod_auth_openidc masking the Keystone federation authentication
endpoint, which results in incorrect responses to requests for
Keystone tokens. This change updates the documentation to
recommend using a vanity URL that does not match a Keystone
API endpoint.
Closes-Bug: 2075349
Change-Id: I1dfba5c71da68522fdb6059f0dc03cddc74cb07d