docs: update internal ALB-related pages (#5491)

Related: #5490.

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site/content/docs/concepts/environments.en.md

@@ -46,9 +46,10 @@ Optionally, when you set up an application, you can provide a domain name that y
 
 An internal ALB is created when a Backend Service with [`http`](../manifest/backend-service.en.md#http) configured in its manifest is deployed in an environment. For an HTTPS endpoint, use the [`--import-cert-arns`](../commands/env-init.en.md#what-are-the-flags) flag when running `copilot env init` and import a VPC with only private subnets. For more on internal ALBs, go [here](../developing/internal-albs.en.md).
 
-If you already have a public-facing ALB in your VPC and would like to use it instead of letting Copilot create one, specify that ALB by name or ARN in your Load-Balanced Web Service's manifest before deploying it to the environment:
+If you already have an ALB in your VPC and would like to use it instead of letting Copilot create one, specify that ALB by name or ARN in your Load-Balanced Web Service's (for internet-facing ALBs) or Backend Service's (for internal ALBs) manifest before deploying it to the environment:
 ```yaml
 http:
+  path: '/'
   alb: [name or ARN]
 ```
 Your imported ALB will be associated with only that service (and any others that similarly import it), rather than shared among all load-balanced services in the environment.  

site/content/docs/developing/internal-albs.en.md

@@ -28,6 +28,12 @@ network:
   vpc:
     placement: private
 ```
+If you have an existing internal ALB in the VPC to which your service will be deployed, you may import it per Backend Service by specifying it in your manifest before deployment:
+```yaml
+http:
+  path: '/'
+  alb: [name or ARN]
+```
 
 ## Advanced Configuration
 

site/content/docs/manifest/backend-service.en.md

@@ -184,6 +184,9 @@ The http section contains parameters related to integrating your service with an
 <span class="parent-field">http.</span><a id="http-path" href="#http-path" class="field">`path`</a> <span class="type">String</span>  
 Requests to this path will be forwarded to your service. Each Backend Service should listen on a unique path.
 
+<span class="parent-field">http.</span><a id="http-alb" href="#http-alb" class="field">`alb`</a> <span class="type">String</span> <span class="version">Added in [v1.33.0](../../blogs/release-v133.en.md#imported-albs)</span>  
+The ARN or name of an existing internal ALB to import. Listener rules will be added to your listener(s). Copilot will not manage DNS-related resources like certificates.
+
 {% include 'http-healthcheck.en.md' %}
 
 <span class="parent-field">http.</span><a id="http-deregistration-delay" href="#http-deregistration-delay" class="field">`deregistration_delay`</a> <span class="type">Duration</span>