Clarify rebase action and new late content additions

Why is this change being made?

Adding clarifications to the release post process to:

1. Update handbook to make clear they are new late additions
2. Update the MR description to make clear that revisions for content in the release post branch should be made with new MRs targeted to the release post branch
3. Update release post manager guidance to avoid rebasing the release post branch against master until after the 0000 yyyy-mm-21 UTC to avoid other pipeline failures that then will require manual intervention by the technical advisor.

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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