Password reset: add documentation of new environment variables

[angela-tran]

From https://github.com/cal-itp/benefits/pull/3293/files#diff-088d9f35d23a4347d221d71dd49b02b95001dff4abe637a40fe0bc04d502049cR13-R15, we have 2 new environment variables related to using an Azure email service.

# Email sending
AZURE_COMMUNICATION_CONNECTION_STRING=
DEFAULT_FROM_EMAIL=noreply@example.calitp.org

Let's update the environment variables Admin interface reference page (https://docs.calitp.org/benefits/reference/admin-interface/) to include these and brief explanation on what they're for.

Acceptance Criteria

• Admin interface reference page contains the above variables and brief explanation on what they're for.

Additional context

• Feat: Azure Communication and Email services #3293
• Existing admin users can initiate account password reset #2940 (epic)