Rearranged install Istio section per review.

Author: erabil-mdb

source/includes/facts/fact-istio.rst

@@ -1,4 +1,4 @@
-We offer the :github:`install_istio_separate_network example script
+In addition, we offer the :github:`install_istio_separate_network example script
 </mongodb/mongodb-enterprise-kubernetes/blob/master/tools/multicluster/install_istio_separate_network.sh>`.
 This script is based on Istio documentation and provides an example installation
 that uses the `multi-primary mode on different networks <https://istio.io/latest/docs/setup/install/multicluster/multi-primary_multi-network/>`__.

source/multi-cluster-prerequisites.txt

@@ -174,16 +174,16 @@ connectivity requirements in the following ways, depending on the type of deploy
 
 .. _install-istio:
 
-Install Istio
-~~~~~~~~~~~~~
+Optional: Install Istio
+~~~~~~~~~~~~~~~~~~~~~~~
 
-Istio is a service mesh that simplifies |dns| resolution and helps establish
+You only need to install |istio| if you :ref:`plan to use a service mesh <mc_plan_external_connectivity>` and Istio is your preferred service mesh provider. Istio is a service mesh that simplifies |dns| resolution and helps establish
 inter-cluster communication between the member |k8s| clusters in a |multi-cluster|.
 If you choose to use a service mesh, you need to install it. If you can't
 utilize a service mesh, skip this section and :ref:`use external domains
 and configure DNS to enable external connectivity <dns-and-external-domain>`.
 
-Install |istio| only if you chose to use it. To learn more, see :ref:`mc_plan_external_connectivity`
+Install |istio| in a `multi-primary mode on different networks <https://istio.io/latest/docs/setup/install/multicluster/multi-primary_multi-network/>`__, using the Istio documentation.
 
 .. include:: /includes/facts/fact-istio.rst