Update attachment create API document

Add 'mode' parameter to request body of
attachment create API.

Change-Id: I109e26837f107d2060f2c2f0ba9c94488e76b730

api-ref/source/v3/attachments.inc

@@ -256,6 +256,7 @@ Request
    - instance_uuid: instance_uuid_1
    - connector: connector
    - volume_uuid: volume_id_7
+   - mode: attach_mode_1
 
 Request Example
 ---------------

api-ref/source/v3/parameters.yaml

@@ -444,6 +444,14 @@ attach_mode:
   in: body
   required: true
   type: string
+attach_mode_1:
+  description: |
+    The attach mode of attachment, acceptable values are
+    read-only ('ro') and read-and-write ('rw').
+  in: body
+  required: false
+  type: string
+  min_version: 3.54
 attach_status:
   description: |
     The volume attach status.