man: Enahnce block_unlisted_ips description

Thanks Christine Caulfield <ccaulfie@redhat.com> for
Englishify and refining the description.

Signed-off-by: Jan Friesse <jfriesse@redhat.com>
Reviewed-by: Christine Caulfield <ccaulfie@redhat.com>

man/corosync.conf.5

@@ -572,6 +572,14 @@ Allow UDPU and KNET to drop packets from IP addresses that are not known
 (nodes which don't exist in the nodelist) to corosync.
 Value is yes or no.
 
+This feature is mainly to protect against the joining of nodes
+with outdated configurations after a cluster split.
+Another use case is to allow the atomic merge of two independent clusters.
+
+Changing the default value is not recommended, the overhead is tiny and
+an existing cluster may fail if corosync is started on an unlisted node
+with an old configuration.
+
 The default value is yes.
 
 .PP