Clean up module documentation (#36909)

* Clean up module documentation

This PR includes:
- Removal of `default: None` (and variations)
- Removal of `required: false`
- Fixing booleans and `type: bool` where required

* Fix remaining (new) validation issues

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

@@ -27,13 +27,11 @@ options:
      description:
         - Indicate desired state of the resource
      choices: ['present', 'absent']
-     required: false
      default: present
    network_name:
      description:
         - Name of the network to which the subnet should be attached
         - Required when I(state) is 'present'
-     required: false
    name:
      description:
        - The name of the subnet that should be created. Although Neutron
@@ -45,78 +43,58 @@ options:
         - The CIDR representation of the subnet that should be assigned to
           the subnet. Required when I(state) is 'present' and a subnetpool
           is not specified.
-     required: false
-     default: None
    ip_version:
      description:
         - The IP version of the subnet 4 or 6
-     required: false
      default: 4
    enable_dhcp:
      description:
         - Whether DHCP should be enabled for this subnet.
-     required: false
-     default: true
+     type: bool
+     default: 'yes'
    gateway_ip:
      description:
         - The ip that would be assigned to the gateway for this subnet
-     required: false
-     default: None
    no_gateway_ip:
      description:
         - The gateway IP would not be assigned for this subnet
-     required: false
-     default: false
+     type: bool
+     default: 'no'
      version_added: "2.2"
    dns_nameservers:
      description:
         - List of DNS nameservers for this subnet.
-     required: false
-     default: None
    allocation_pool_start:
      description:
         - From the subnet pool the starting address from which the IP should
           be allocated.
-     required: false
-     default: None
    allocation_pool_end:
      description:
         - From the subnet pool the last IP that should be assigned to the
           virtual machines.
-     required: false
-     default: None
    host_routes:
      description:
         - A list of host route dictionaries for the subnet.
-     required: false
-     default: None
    ipv6_ra_mode:
      description:
         - IPv6 router advertisement mode
      choices: ['dhcpv6-stateful', 'dhcpv6-stateless', 'slaac']
-     required: false
-     default: None
    ipv6_address_mode:
      description:
         - IPv6 address mode
      choices: ['dhcpv6-stateful', 'dhcpv6-stateless', 'slaac']
-     required: false
-     default: None
    use_default_subnetpool:
      description:
         - Use the default subnetpool for I(ip_version) to obtain a CIDR.
-     required: false
-     default: false
+     type: bool
+     default: 'no'
    project:
      description:
         - Project name or ID containing the subnet (name admin-only)
-     required: false
-     default: None
      version_added: "2.1"
    availability_zone:
      description:
        - Ignored. Present for backwards compatibility
-     required: false
 requirements:
     - "python >= 2.6"
     - "shade"