Document GCP firewall allowing inter-VPC traffic (#2563)

Author: r4victor

docs/docs/concepts/backends.md

@@ -529,7 +529,11 @@ gcloud projects list --format="json(projectId)"
         ```
 
         </div>
-    
+
+        If you specify a non-default VPC, ensure it has a firewall rule
+        allowing all traffic within the VPC. This is needed for multi-node tasks to work.
+        The default VPC already permits traffic within the VPC.
+
     === "Shared VPC"
 
         <div editor-title="~/.dstack/server/config.yml">

examples/misc/a3high-clusters/README.md

@@ -51,6 +51,11 @@ projects:
 
 </div>
 
+!!! info "Custom VPC"
+    If you specify a non-default primary VPC, ensure it has a firewall rule
+    allowing all traffic within the VPC. This is needed for MPI and NCCL to work.
+    The default VPC already permits traffic within the VPC.
+
 ??? info "Create extra VPCs"
     Create the VPC networks for GPUDirect in your project, each with a subnet and a firewall rule:
     

examples/misc/a3mega-clusters/README.md

@@ -45,6 +45,11 @@ projects:
 
 </div>
 
+!!! info "Custom VPC"
+    If you specify a non-default primary VPC, ensure it has a firewall rule
+    allowing all traffic within the VPC. This is needed for MPI and NCCL to work.
+    The default VPC already permits traffic within the VPC.
+
 ??? info "Create extra VPCs"
     Create the VPC networks for GPUDirect in your project, each with a subnet and a firewall rule: