fix documentation of mtu field in interconnect attachments resource (GoogleCloudPlatform#14053)

Author: harshithpatte-g

mmv1/products/compute/InterconnectAttachment.yaml

@@ -108,8 +108,8 @@ properties:
   - name: 'mtu'
     type: String
     description: |
-      Maximum Transmission Unit (MTU), in bytes, of packets passing through
-      this interconnect attachment. Currently, only 1440 and 1500 are allowed. If not specified, the value will default to 1440.
+      Maximum Transmission Unit (MTU), in bytes, of packets passing through this interconnect attachment.
+      Valid values are 1440, 1460, 1500, and 8896. If not specified, the value will default to 1440.
     default_from_api: true
     custom_flatten: 'templates/terraform/custom_flatten/float64_to_int_to_string.go.tmpl'
   - name: 'bandwidth'