Enhance docs on authorization realms

[jkakavas]

In configuring-authorization-delegation.asciidoc, instead of examples we could have a section for each realm that can act as an authorization realm with the necessary configuration and caveats

i.e. The fact that LDAP and AD realms need to be configured with a bind_dn user is implied in other parts of the documentation where we say that only realms that support lookup can be used as authorization realms but it might not be obvious to make the connection that an LDAP/AD realm needs the bind_dn user in order to perform lookups.

[elasticmachine]

Pinging @elastic/es-security (:Security/Authentication)

[elasticmachine]

Pinging @elastic/es-docs (>docs)

[tvernum]

We have a rough explanation for this under run-as.
https://www.elastic.co/guide/en/elasticsearch/reference/current/run-as-privilege.html#run-as-privilege

We could copy that text, but perhaps we need a section that explains "looking up users by principal" and link to it from both run-as and authz delegation.