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/amazon/redshift_subnet_group.py

@@ -35,14 +35,10 @@ options:
   group_description:
     description:
       - Database subnet group description.
-    required: false
-    default: null
     aliases: ['description']
   group_subnets:
     description:
       - List of subnet IDs that make up the cluster subnet group.
-    required: false
-    default: null
     aliases: ['subnets']
 requirements: [ 'boto' ]
 extends_documentation_fragment: