chore(backport release-1.3): docs: note kustomize-set-image tag/digest requirement

[akuitybot]

Automated backport to release-1.3, triggered by a label in #3581.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 52.63%. Comparing base (25fb512) to head (93df143).
Report is 1 commits behind head on release-1.3.

Additional details and impacted files

@@             Coverage Diff              @@
##           release-1.3    #3583   +/-   ##
============================================
  Coverage        52.63%   52.63%           
============================================
  Files              296      296           
  Lines            26881    26881           
============================================
  Hits             14150    14150           
  Misses           11957    11957           
  Partials           774      774           

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