docs: Clarify merge process

[podarok]

Related Issue/Ticket:

PLEASE CHECK BASE BRANCH FOR YOUR PR
ONLY urgent and approved fixes for point release should go to master branch

(Replace this line with a link to the related GitHub issue on ymcatwincities/openy_gated_content or your own ticketing system)

Steps to test:

• First do this
• Then do this (and look at this great screenshot).
• Finally observe the issue is resolved.

Quality checks:

Please check these boxes to confirm this PR covers the following cases:

• Maintaining our upgrade path is essential. Check one or the other:
  • This PR provides updates via hook_update_N or other means.
  • No updates are necessary for this change.
• Front end fixes should be tested against all of the Open Y Themes.
  • Tested against Carnation
  • Tested against Lily
  • Tested against Rose
  • This change does not contain front-end fixes.
• I have flagged this PR "Needs Review" or pinged the VY devs/QA
  team in Slack

[fjbot]

Build 4393

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