Update release instructions for merging branches

[jeffpaul]

What?

This pull request makes a small update to the release instructions in docs/RELEASE_INSTRUCTIONS.md to clarify and separate the merge steps during a release. The instructions now explicitly distinguish merging the release PR into develop and then merging develop into trunk.

Why?

Because I somehow flipped those steps in the 0.4.0 release ( 🤦🏼 ), see #265 (comment).

How?

Use of AI Tools

Testing Instructions

Testing Instructions for Keyboard

Screenshots or screencast

Before	After

[github-actions]

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Co-authored-by: jeffpaul <jeffpaul@git.wordpress.org>

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 57.68%. Comparing base (d9bd308) to head (25786dd).
⚠️ Report is 2 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff             @@
##             develop     #277   +/-   ##
==========================================
  Coverage      57.68%   57.68%           
  Complexity       567      567           
==========================================
  Files             35       35           
  Lines           2933     2933           
==========================================
  Hits            1692     1692           
  Misses          1241     1241           

Flag	Coverage Δ
unit	57.68% <ø> (ø)

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