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.

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

  • 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.

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