mgr: add doc to describe relationship to prometheus

mgr: add doc to describe relationship to prometheus

The healthcheck for slow_ops is used by the mgr/prometheus
module so it's important the alert generated matches the
format expected in mgr/prometheus.

Signed-off-by: Paul Cuzner <pcuzner@redhat.com>
(cherry picked from commit fc1905b4a74aedc6bdbc928b6d6e514183bbf090)

Conflicts:
	src/mgr/DaemonHealthMetricCollector.cc

- docstring added to describe the link to mgr/prometheus conflicted with the
  const fmt definition for the message. resolved by adding doc under the const
  definition.