Adding clarification you can expense phone costs in the month you are on-call

Why is this change being made?

To remain consistent with the policy used in Engineering adding clarification around that you can expense phone costs in the month you are on-call.

Support Engineering policy: https://about.gitlab.com/handbook/support/on-call/#mobile-phone-service-and-data-reimbursement

Slack thread: https://gitlab.slack.com/archives/C14FJAYD7/p1624297434164700?thread_ts=1624291058.161300&cid=C14FJAYD7

For your review @brentnewton @tcooney @jurbanc @cdu1 @eboyd1

@eboyd1 Could you help tag the right person in Finance for review?

Author Checklist

• Provided a concise title for the MR
• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
• Assign this change to the correct DRI
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