Update JDBCConfig documentation to warn of security risks

The password in jdbcconfig.properties is unencrypted and using the password encryption mechanism that's normally used for datastores, etc doesn't work due to circular dependencies.   (The password is needed to get the configuration, but the configuration is needed to set up the key store needed to decrypt the password)

JNDI avoids this issue by offloading the problem of securing the password to the application server. Documenting that direct connection is provided as a convenience and has security implications which can be avoided with JNDI seems the best resolution.