Clean up module documentation (#36909)

* Clean up module documentation

This PR includes:
- Removal of `default: None` (and variations)
- Removal of `required: false`
- Fixing booleans and `type: bool` where required

* Fix remaining (new) validation issues

lib/ansible/modules/cloud/google/gcp_backend_service.py

@@ -46,13 +46,10 @@ options:
   enable_cdn:
     description:
        - If true, enable Cloud CDN for this Backend Service.
-    required: false
   port_name:
     description:
       - Name of the port on the managed instance group (MIG) that backend
         services can forward data to. Required for external load balancing.
-    required: false
-    default: null
   protocol:
     description:
        - The protocol this Backend Service uses to communicate with backends.
@@ -66,22 +63,15 @@ options:
   service_account_email:
     description:
       - Service account email
-    required: false
-    default: null
   credentials_file:
     description:
       - Path to the JSON file associated with the service account email.
-    default: null
-    required: false
   project_id:
     description:
       - GCE project ID.
-    required: false
-    default: null
   state:
     description:
       - Desired state of the resource
-    required: false
     default: "present"
     choices: ["absent", "present"]
 '''