Document Helm image reference strategies

[briandealwis]

Description
Improve Helm docs by updating the image reference strategies. Some of the text seems to pre-date the new helm strategy.

[codecov]

Codecov Report

Merging #6017 (0d7c48f) into master (7f754f6) will decrease coverage by 0.07%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #6017      +/-   ##
==========================================
- Coverage   70.79%   70.72%   -0.08%     
==========================================
  Files         462      462              
  Lines       17866    17887      +21     
==========================================
+ Hits        12649    12651       +2     
- Misses       4286     4304      +18     
- Partials      931      932       +1     

Impacted Files	Coverage Δ
pkg/skaffold/util/tar.go	50.66% <0.00%> (-5.34%)	⬇️
pkg/diag/validator/pod.go	1.11% <0.00%> (-0.12%)	⬇️
pkg/skaffold/deploy/status/status_check.go	70.92% <0.00%> (+0.84%)	⬆️
pkg/skaffold/docker/parse.go	87.14% <0.00%> (+0.95%)	⬆️

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[container-tools-bot]

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