Clarify allowed_pull_policies default

What does this MR do?

If allowed_pull_policies is not specified in the configuration, it will be equivalent to the specified pull_policy. This update clarifies that.

Why was this MR needed?

US Gov customer (ticket) had issues with conflicting configurations, because they did not understand the allowed_pull_policies keyword has a default.

What's the best way to test this MR?

What's the best way to test this MR?

What are the relevant issue numbers?