Add clarity about whether to include deprecation notices in code change MRs

Why is this change being made?

Documentation is included with code change MRs as part of definition of done. Because deprecation posts are a specific type of documentation update, it's not clear if they can be included in a code change or if they must be in a separate MR that uses the deprecation template.

This MR adds clarity to the deprecations guidelines in the handbook so it's clear to the reviewing TW that the deprecation notices must be completed separate to the code change MR.

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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