Fix documentation for gateway_ip in subnet update

The documentation re-used the string for the subnet creation, which has
a different behavior as seen in [1]: on creation, not providing
a gateway_ip lets neutron pick one for us, while for the update, this is
a no-op.

[1] https://github.com/k-orc/openstack-resource-controller/pull/403#discussion_r2139426464

Change-Id: Ied169c8369a1a0bc50d13f6b7ac69bfb8d1c3cee

api-ref/source/v2/parameters.yaml

@@ -7046,6 +7046,13 @@ subnet-gateway_ip-request:
   in: body
   required: false
   type: string
+subnet-gateway_ip-update-request:
+  description: |
+    Gateway IP of this subnet. If the value is ``null`` that implies no
+    gateway is associated with the subnet.
+  in: body
+  required: false
+  type: string
 subnet-host_routes:
   description: |
     Additional routes for the subnet. A list of dictionaries with

api-ref/source/v2/subnets.inc

@@ -463,7 +463,7 @@ Request
    - dns_nameservers: subnet-dns_nameservers-request
    - allocation_pools: subnet-allocation_pools-request
    - host_routes: subnet-host_routes-request
-   - gateway_ip: subnet-gateway_ip-request
+   - gateway_ip: subnet-gateway_ip-update-request
    - description: description-request
    - service_types: subnet-service_types-optional
    - segment_id: subnet-segment_id-request