docs: dev.md improvements

[ahmetb]

• added note about the API (relevant to <1% users)
• attempt to clarify what notify means by replacing it

[codecov]

Codecov Report

Merging #6503 (5599395) into main (a99ffc5) will increase coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #6503      +/-   ##
==========================================
+ Coverage   70.27%   70.30%   +0.03%     
==========================================
  Files         510      512       +2     
  Lines       22988    23031      +43     
==========================================
+ Hits        16154    16192      +38     
- Misses       5776     5781       +5     
  Partials     1058     1058              

Impacted Files	Coverage Δ
pkg/skaffold/log/stream/stream.go	71.42% <0.00%> (-14.29%)	⬇️
pkg/skaffold/test/test_factory.go	76.19% <0.00%> (-1.09%)	⬇️
pkg/skaffold/hooks/deploy.go	85.71% <0.00%> (ø)
pkg/skaffold/runner/v1/test.go	100.00% <0.00%> (ø)
pkg/skaffold/hooks/types.go	75.00% <0.00%> (ø)
pkg/skaffold/deploy/helm/deploy.go	77.91% <0.00%> (+1.09%)	⬆️
pkg/skaffold/deploy/kustomize/kustomize.go	74.51% <0.00%> (+1.98%)	⬆️

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