This is useful in cases where the caller of the API needs to know
the name of the realm that consumed the SAML Response and
authenticated the user and this is not self evident (i.e. because
there are many saml realms defined in ES).
Currently, the way to learn the realm name would be to make a
subsequent request to the `_authenticate` API.
This change adds documentation for the SAML APIs in Elasticsearch
and adds simple instructions on how these APIs can be used to
authenticate a user with SAML by a custom web application other
than Kibana.
Resolves: #40352