doc: update offer_url use in integration resource

Per juju#449, indicate that offer_url and name/endpoint are mutually
exclusive in an integration resource.

examples/resources/juju_offer/resource.tf

@@ -1,19 +1,18 @@
-resource "juju_offer" "this" {
+resource "juju_offer" "myoffer" {
   model            = juju_model.development.name
   application_name = juju_application.percona-cluster.name
   endpoint         = server
 }
 
-// an offer can then be used in an integration as below:
-resource "juju_integration" "this" {
+// an offer can then be used in an cross model integration as below:
+resource "juju_integration" "myintegration" {
   model = juju_model.development-destination.name
 
   application {
     name     = juju_application.wordpress.name
     endpoint = "db"
   }
-
   application {
-    offer_url = juju_offer.this.url
+    offer_url = juju_offer.myoffer.url
   }
 }

internal/provider/resource_integration.go

@@ -10,6 +10,7 @@ import (
 	"strings"
 
 	"github.com/hashicorp/terraform-plugin-framework-validators/setvalidator"
+	"github.com/hashicorp/terraform-plugin-framework-validators/stringvalidator"
 	"github.com/hashicorp/terraform-plugin-framework/diag"
 	"github.com/hashicorp/terraform-plugin-framework/path"
 	"github.com/hashicorp/terraform-plugin-framework/resource"
@@ -138,20 +139,33 @@ func (r *integrationResource) Schema(_ context.Context, _ resource.SchemaRequest
 				NestedObject: schema.NestedBlockObject{
 					Attributes: map[string]schema.Attribute{
 						"name": schema.StringAttribute{
-							Description: "The name of the application.",
-							Optional:    true,
+							Description: "The name of the application. This attribute may not be used at the" +
+								" same time as the offer_url.",
+							Optional: true,
+							Validators: stringvalidator.ConflictsWith(
+								path.MatchRelative().AtParent().AtName("offer_url"),
+							),
 						},
 						"endpoint": schema.StringAttribute{
-							Description: "The endpoint name.",
-							Optional:    true,
-							Computed:    true,
+							Description: "The endpoint name. This attribute may not be used at the" +
+								" same time as the offer_url.",
+							Optional: true,
+							Computed: true,
+							Validators: stringvalidator.ConflictsWith(
+								path.MatchRelative().AtParent().AtName("offer_url"),
+							),
 						},
 						"offer_url": schema.StringAttribute{
-							Description: "The URL of a remote application.",
-							Optional:    true,
+							Description: "The URL of a remote application. This attribute may not be used at the" +
+								"same time as name and endpoint.",
+							Optional: true,
 							PlanModifiers: []planmodifier.String{
 								stringplanmodifier.UseStateForUnknown(),
 							},
+							Validators: stringvalidator.ConflictsWith(
+								path.MatchRelative().AtParent().AtName("name"),
+								path.MatchRelative().AtParent().AtName("endpoint"),
+							),
 						},
 					},
 				},