emphasize the folllowing cluster need to have correct version

I do believe a new version compatibility matrix should be made for following index. 
But it might involve other issues that support not thinking about. 
The current matrix here would likely mislead the customer to think the following cluster can be older than the remote cluster.

Author: Camilleli

docs/reference/ccr/index.asciidoc

@@ -23,7 +23,7 @@ leader indices, and the other cluster contains only follower indices.
 * In a bi-directional configuration, each cluster contains both leader and
 follower indices.
 
-In a uni-directional configuration, the cluster containing follower indices
+IMPORTANT: In a uni-directional configuration, the cluster containing follower indices
 must be running **the same or newer** version of {es} as the remote cluster.
 If newer, the versions must also be compatible as outlined in the following matrix.