NO-JIRA: HA documentation improvements.

The broker options link-maintenance-interval and link-heartbeat-interval were documented
only in the HA section and not in the federation section which created confusion about
whether they affected HA links only or user federation links as well.

Added documentation to the federation section and an explicit mention in the HA section
that it uses federation links.

git-svn-id: https://svn.apache.org/repos/asf/qpid/trunk@1618160 13f79535-47bb-0310-9956-ffa450edef68