Update ListUsers.py to include Pagination in the Example

[sadhbh-a]

What does this PR do?

Add a line on pagination to the code example & clarifies that the USER_DATA_ATTRIBUTES_EMAIL needs to be set locally in order to populate (I think this is useful to note as while testing I kept running into this issue)

We have received multiple tickets in the past for customers confused on why when they run the below command: curl -X GET "https://api.datadoghq.eu/api/v2/users" \ only 10 users users are returned. The reason for this is that this command requires pagination. I think having this in the snippet would help avoid these kinds of tickets.

Examples: https://datadog.zendesk.com/agent/tickets/1753134 https://datadog.zendesk.com/agent/tickets/1363875 https://datadog.zendesk.com/agent/tickets/1090161 https://datadog.zendesk.com/agent/tickets/1470738

Additional Notes

Review checklist

Please check relevant items below:

• [ ] This PR includes all newly recorded cassettes for any modified tests.

• [x] This PR does not rely on API client schema changes.

  • [x] The CI should be fully passing.

• [ ] Or, this PR relies on API schema changes and this is a Draft PR to include tests for that new functionality.

  • Note: CI shouldn't be run on this Draft PR, as its expected to fail without the corresponding schema changes.