Add some documentation of datasource authentication

The datasource docs in the Admin Guide[1] could use documentation on how to configure authentication to the database. What's there now just has some example config snippets that use a statically configured cleartext username and password. Integration with the appserver security layer is one of the more user-visible things the datasource subsystem provides, so our docs should cover it a bit.

• Use of a credential reference.
• Use of an Elytron authentication context.

To me getting into too much detail of security configuration seems beyond the scope of the Admin Guide datasource doc section so probably the datasource doc should link out to security docs like [2] for further detail. If there aren't appropriate things to link to a blocking task would be for someone to add them.

[1] https://docs.wildfly.org/27/Admin_Guide#DataSource
[2] https://docs.wildfly.org/27/WildFly_Elytron_Security.html