fix: document and add 'accept-ra' to network schema (#5060)

It has been available in v1 config since 62bbc26

cloudinit/config/schemas/schema-network-config-v1.json

@@ -36,6 +36,10 @@
           "items": {
             "$ref": "#/$defs/config_type_subnet"
           }
+        },
+        "accept-ra": {
+          "type": "boolean",
+          "description": "Whether to accept IPv6 Router Advertisements (RA) on this interface. If unset, it will not be rendered"
         }
       }
     },

doc/rtd/reference/network-config-format-v1.rst

@@ -82,6 +82,13 @@ be sent in a packet- or frame-based network. Specifying ``mtu`` is optional.
    configuration time. It's possible to specify a value too large or to
    small for a device, and may be ignored by the device.
 
+``accept-ra: <boolean>``
+^^^^^^^^^^^^^^^^^^^^^^^^
+
+The ``accept-ra`` key is a boolean value that specifies whether or not to
+accept Router Advertisements (RA) for this interface. Specifying ``accept-ra``
+is optional.
+
 Physical example
 ^^^^^^^^^^^^^^^^