Caveat auto issue closing in handbook

Why is this change being made?

Automatic issue closing can be disabled (and certainly is on at least one major project: gitlab-org/gitlab).

It seems like we should either remove or caveat:

Pro Tip: When creating a Merge Request you can add closes: #[insert issue number here] and when the Merge Request is merged, the issue will automatically close. You can see an example of this here.

From https://about.gitlab.com/handbook/communication/#issues

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.
• 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
• 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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