[EDIT] Provide more information about cluster-health colors up front

I don't want to go into any more detail at this point because we
haven't yet explained shards.

Author: clintongormley

020_Distributed_Cluster/10_Cluster_health.asciidoc

@@ -29,6 +29,13 @@ which, on an empty cluster with no indices, will return something like:
 --------------------------------------------------
 <1> The `status` field is the one we're most interested in.
 
-We will explain the meaning of each of these colors in the relevant section
-below.
+The `status` field provides an overall indication of how the cluster is
+functioning. The meaning of the three colors are provided here for reference:
 
+[horizontal]
+`green`::   All primary and replica shards are active.
+`yellow`::  All primary shards are active, but not all replica shards are active.
+`red`::     Not all primary shards are active.
+
+In the rest of this chapter we explain what _primary_ and _replica_ shards are
+and explain the practical implications of each of the above colors.