Add Docs and Feature Flags to work type classification definitions

Why is this change being made?

In the FY23 MR Data Audit, we needed to determine whether the work type label on the MR was the correct label to be on the MR. What we found was that in the cases of Feature Flags or Documentation, there were major inconsistencies:

• If done as part of a feature issue - sometimes feature, sometimes maintenance
• If done as part of a bug issue - sometimes bug, sometimes maintenance
• If done just because - usually maintenance

This MR provides clarification to all team members that a feature flag cleanup or documentation update on it's own, or rather without an actual feature or bug fix included, should be considered maintenance.

Author Checklist

• Provided a concise title for this Merge Request (MR)
• Added a description to this MR explaining the reasons for the proposed change, per say why, not just what
  • Copy/paste the Slack conversation to document it for later, or upload screenshots. Verify that no confidential data is added, and the content is SAFE
• Assign reviewers for this MR to the correct Directly Responsible Individual/s (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 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
  • The when to get approval handbook section explains the workflow in more detail
• If the changes affect team members, or warrant an announcement in another way, please consider posting an update in #whats-happening-at-gitlab linking to this MR
  • If this is a change that directly impacts the majority of global team members, it should be a candidate for #company-fyi. Please work with internal communications and check the handbook for examples.

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