dcache alarms: add missing configuration elements and documentation for SMTP appender

This is a minimal backport of http://rb.dcache.org/r/6949.

Instead of adding support for exposing the entire configuration
of the SMTP appender through the dcache property framework,
we have simply added the missing elements, plus explanatory
comments, to the logback-server.xml.

Target: 2.9
Request: 2.8
Request: 2.7
Request: 2.6
Patch: http://rb.dcache.org/r/6957
Acked-by: Paul
Merge pull request #525 from alrossi/fix/2.9/alarms-email-configuration

Fix/2.9/alarms email configuration