docs: clarify that tagged images in manifests are not replaced

[corneliusweig]

Related #2547

[codecov]

Codecov Report

Merging #2598 into master will decrease coverage by 0.53%.
The diff coverage is n/a.

Impacted Files	Coverage Δ
cmd/skaffold/app/cmd/config/unset.go	15.38% <0%> (-67.95%)	⬇️
cmd/skaffold/app/cmd/config/list.go	0% <0%> (-65.22%)	⬇️
...ffold/kubernetes/portforward/resource_forwarder.go	56% <0%> (-32%)	⬇️
cmd/skaffold/app/cmd/config/set.go	71.01% <0%> (-13.2%)	⬇️
pkg/skaffold/deploy/util.go	68% <0%> (-10.95%)	⬇️
pkg/skaffold/runner/runcontext/context.go	62.5% <0%> (-9.6%)	⬇️
pkg/skaffold/initializer/init.go	53.6% <0%> (-7.69%)	⬇️
pkg/skaffold/trigger/triggers.go	30.95% <0%> (-2.75%)	⬇️
pkg/skaffold/deploy/helm.go	64.9% <0%> (-1.64%)	⬇️
pkg/skaffold/runner/deploy.go	70.58% <0%> (-1.64%)	⬇️
... and 42 more

[balopat]

LGTM