Fix docs and error message about pullSecretPath

[chanseokoh]

The property was renamed to pullSecretPath in #4147 and now serves as a dual-purpose property:

• Sometimes used to specify a path to a local secret file.
• Other times only used for the mount path of a secret file inside a Kaniko pod.

[codecov]

Codecov Report

Merging #4381 into master will decrease coverage by 0.01%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master    #4381      +/-   ##
==========================================
- Coverage   71.78%   71.77%   -0.02%     
==========================================
  Files         325      325              
  Lines       12594    12594              
==========================================
- Hits         9041     9039       -2     
- Misses       2978     2979       +1     
- Partials      575      576       +1     

Impacted Files	Coverage Δ
pkg/skaffold/schema/defaults/defaults.go	90.72% <0.00%> (ø)
pkg/skaffold/build/cluster/secret.go	41.53% <100.00%> (ø)
...affold/kubernetes/portforward/kubectl_forwarder.go	60.97% <0.00%> (-2.44%)	⬇️

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