[release/1.16.0] api-gateway: update docs (#17884)

* api-gateway: add operation cannot be fulfilled error to common errors (#17874)

* add error message

* Update website/content/docs/api-gateway/usage/errors.mdx

Co-authored-by: Nathan Coleman <nathan.coleman@hashicorp.com>

* fix formating issues

---------

Co-authored-by: Nathan Coleman <nathan.coleman@hashicorp.com>

* api-gateway: add step to upgrade instructions for creating intentions (#17875)

---------

Co-authored-by: sarahalsmiller <100602640+sarahalsmiller@users.noreply.github.com>

website/content/docs/api-gateway/upgrades.mdx

@@ -65,6 +65,8 @@ If you are able to tolerate downtime for your applications, you should delete pr
   $ kubectl apply -f apigw-installation.yaml
   ```
 
+1. Create `ServiceIntentions` allowing `Gateways` to communicate with any backend services that they route to. Refer to [Service intentions configuration entry reference](/consul/docs/connect/config-entries/service-intentions) for additional information.
+
 1. Change any existing `Gateways` to reference the new `GatewayClass` `consul`. Refer to [gatewayClass](/consul/docs/api-gateway/configuration/gateway#gatewayclassname) for additional information.
 
 1. After updating all of your `gateway` configurations to use the new controller, you can complete the upgrade again and completely remove the `apiGateway` block to remove the old controller. 
@@ -99,6 +101,8 @@ If you are unable to tolerate any downtime, you can complete the following steps
   $ kubectl apply -f apigw-installation.yaml
   ```
 
+1. Create `ServiceIntentions` allowing `Gateways` to communicate with any backend services that they route to. Refer to [Service intentions configuration entry reference](/consul/docs/connect/config-entries/service-intentions) for additional information.
+
 1. Change any existing `Gateways` to reference the new `GatewayClass` `consul`. Refer to [gatewayClass](/consul/docs/api-gateway/configuration/gateway#gatewayclassname) for additional information.
 
 

website/content/docs/api-gateway/usage/errors.mdx

@@ -58,3 +58,18 @@ The installation process typically fails after this error message is generated.
 
 **Resolution:**
 Install the required CRDs. Refer to the [Consul API Gateway installation instructions](/consul/docs/api-gateway/install#installation) for instructions.
+
+## Operation cannot be fulfilled, the object has been modified
+
+```
+{"error": "Operation cannot be fulfilled on gatewayclassconfigs.consul.hashicorp.com \"consul-api-gateway\": the object has been modified; please apply your changes to the latest version and try again"}
+
+```
+**Conditions:**
+This error occurs when the gateway controller attempts to update an object that has been modified previously. It is a normal part of running the controller and will resolve itself by automatically retrying.
+
+**Impact:**
+Excessive error logs are produced, but there is no impact to the functionality of the controller.
+
+**Resolution:**
+No action needs to be taken to resolve this issue.