Adding GCP XPN mark for installation support table

Author: codyhoag

installing/installing-preparing.adoc

@@ -228,4 +228,15 @@ endif::openshift-origin[]
 |xref:../installing/installing_ibm_z/installing-restricted-networks-ibm-z.adoc#installing-restricted-networks-ibm-z[X]
 |xref:../installing/installing_ibm_power/installing-restricted-networks-ibm-power.adoc#installing-restricted-networks-ibm-power[X]
 
+|Shared VPC hosted outside of cluster project
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+|xref:../installing/installing_gcp/installing-gcp-user-infra-vpc.adoc#installing-gcp-user-infra-vpc[X]
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 |===

installing/installing_gcp/installing-gcp-user-infra-vpc.adoc

@@ -5,8 +5,7 @@ include::modules/common-attributes.adoc[]
 
 toc::[]
 
-In {product-title} version {product-version}, you can install a cluster into a shared Virtual Private Cloud (VPC) on
-Google Cloud Platform (GCP) that uses infrastructure that you provide.
+In {product-title} version {product-version}, you can install a cluster into a shared Virtual Private Cloud (VPC) on Google Cloud Platform (GCP) that uses infrastructure that you provide. In this context, a cluster installed into a shared VPC is a cluster that is configured to use a VPC from a project different from where the cluster is being deployed. For more information, see Google's documentation on link:https://cloud.google.com/vpc/docs/shared-vpc#shared_vpc_networks[shared VPC networks].
 
 The steps for performing a user-provided infrastructure installation into a shared VPC are outlined here. Several
 link:https://cloud.google.com/deployment-manager/docs[Deployment Manager] templates are provided to assist in