Make clear from expense policy that not everything is reimbursable

Why is this change being made?

I think that this page could be interpreted by people unfamiliar, that GitLab will always reimburse as long as you use your (personal) judgement. Based on examples in https://about.gitlab.com/handbook/finance/expenses/#-non-reimbursable-expenses, https://about.gitlab.com/handbook/finance/expenses/#subscription-exceptions, and https://about.gitlab.com/handbook/finance/expenses/#-hardware that is not the case. Also e.g.:

If you wish to go over the recommended reimbursement, this is an exception and should first be raised with your manager and if supported, should be raised in the "expense-reporting-inquiries who will review and respond to the request.

Update this page to encourage team members to check the expense policy.

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
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to one of the people listed in the "Maintained by" section in on the page being edited.
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in #mr-buddies.
  • If the changes relate to any part of the project other than updates to content and/or data files please make sure to ping @gl-static-site-editor in a comment for a review and merge. For example changes to .gitlab-ci.yml, JavaScript/CSS/Ruby code or the layout files. (this requirement has been removed pending identification of a new DRI for the handbook)