Allow BackendService backends.group to ref multiple resource types (#151

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<!-- This change is generated by MagicModules. -->
/cc @rileykarson

docs/resources/google_compute_backend_service.md

@@ -35,7 +35,7 @@ Properties that can be accessed from the `google_compute_backend_service` resour
 
     * `description`: An optional description of this resource. Provide this property when you create the resource.
 
-    * `group`: This instance group defines the list of instances that serve traffic. Member virtual machine instances from each instance group must live in the same zone as the instance group itself.  No two backends in a backend service are allowed to use same Instance Group resource.  When the BackendService has load balancing scheme INTERNAL, the instance group must be in a zone within the same region as the BackendService.
+    * `group`: The fully-qualified URL of an Instance Group or Network Endpoint Group resource. In case of instance group this defines the list of instances that serve traffic. Member virtual machine instances from each instance group must live in the same zone as the instance group itself. No two backends in a backend service are allowed to use same Instance Group resource.  For Network Endpoint Groups this defines list of endpoints. All endpoints of Network Endpoint Group must be hosted on instances located in the same zone as the Network Endpoint Group.  Backend service can not contain mix of Instance Group and Network Endpoint Group backends.  Note that you must specify an Instance Group or Network Endpoint Group resource using the fully-qualified URL, rather than a partial URL.  When the BackendService has load balancing scheme INTERNAL, the instance group must be within the same region as the BackendService. Network Endpoint Groups are not supported for INTERNAL load balancing scheme.
 
     * `max_connections`: The max number of simultaneous connections for the group. Can be used with either CONNECTION or UTILIZATION balancing modes.  For CONNECTION mode, either maxConnections or maxConnectionsPerInstance must be set.  This cannot be used for internal load balancing.