Expanded istructions to configure JNDI for WAR installs

Currently provides basic instructions, with examples for setting up a JNDI connection pool with a postgres data source. Provides a link to the geoserver user manual for configuring other data sources (Oracle and SQL Server)

If we want to include instructions for Oracle and SQL Server in suite, I would recomend splitting this section into a seperate JNDI configuration document, or even a seperate document for each of Postgres, Oracle, and SQL Server, because the necessary instructions are rather long, but also have a fair amount of repeated content.