Documentation issues with TEIID-3669

With the consolidation of the session service, we don't have a security-domain per transport. Rather there is a default, which can also be set on a per-vdb basis. The rationale is that establishing a role mapping needs to be consistent about where the enterprise role names are coming from.

The documentation of using other security mechanisms from the web tier needs to be reconciled with this as it currently still documents changing the security domain at a transport level.