Update the wait-for-additional-items design doc link

[emmanuel-ferdman]

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PR #6087 moved the location of wait-for-additional-items.md. This PR adjusts sources to changes.

Does your change fix a particular issue?

Fixes #(issue)

Please indicate you've done the following:

• Accepted the DCO. Commits without the DCO will delay acceptance.
• Created a changelog file (make new-changelog) or comment /kind changelog-not-required on this PR.
• Updated the corresponding documentation in site/content/docs/main.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 59.15%. Comparing base (3f9c2dc) to head (5d0f09d).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #8225   +/-   ##
=======================================
  Coverage   59.15%   59.15%           
=======================================
  Files         367      367           
  Lines       30777    30777           
=======================================
  Hits        18206    18206           
  Misses      11113    11113           
  Partials     1458     1458           

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