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/azure/azure_rm_subnet.py

@@ -42,15 +42,12 @@ options:
     security_group_name:
         description:
             - Name of an existing security group with which to associate the subnet.
-        required: false
-        default: null
         aliases:
             - security_group
     state:
         description:
             - Assert the state of the subnet. Use 'present' to create or update a subnet and
               'absent' to delete a subnet.
-        required: false
         default: present
         choices:
             - absent