Docs: Avoid use of 'default: null' (#45795)

Various modules document the default 'null' value, but it causes None to
be shown in the documentation explicitly.

lib/ansible/modules/cloud/openstack/os_volume_snapshot.py

@@ -2,19 +2,7 @@
 # coding: utf-8 -*-
 
 # Copyright (c) 2016, Mario Santos <mario.rf.santos@gmail.com>
-#
-# This module is free software: you can redistribute it and/or modify
-# it under the terms of the GNU General Public License as published by
-# the Free Software Foundation, either version 3 of the License, or
-# (at your option) any later version.
-#
-# This software is distributed in the hope that it will be useful,
-# but WITHOUT ANY WARRANTY; without even the implied warranty of
-# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
-# GNU General Public License for more details.
-#
-# You should have received a copy of the GNU General Public License
-# along with this software.  If not, see <http://www.gnu.org/licenses/>.
+# GNU General Public License v3.0+ (see COPYING or https://www.gnu.org/licenses/gpl-3.0.txt)
 
 from __future__ import absolute_import, division, print_function
 __metaclass__ = type
@@ -42,8 +30,6 @@ options:
    display_description:
      description:
        - String describing the snapshot
-     required: false
-     default: null
      aliases: ['description']
    volume:
      description:
@@ -53,9 +39,8 @@ options:
      description:
        - Allows or disallows snapshot of a volume to be created when the volume
          is attached to an instance.
-     required: false
-     default: False
      type: bool
+     default: 'no'
    state:
      description:
        - Should the resource be present or absent.
@@ -64,7 +49,6 @@ options:
    availability_zone:
      description:
        - Availability zone in which to create the snapshot.
-     required: false
 requirements:
      - "python >= 2.7"
      - "openstacksdk"