[DOCS] Add comment about passwordPath usage in flyte-core helm chart

[spwoodcock]

Why are the changes needed?

• I just tried to deploy flyte, but the admin and datacatalog containers kept failing at the run-migrations initContainer due to an invalid password.
• After tearing my hair out for a while, and digging into the code here, I finally found the passwordPath param that must be specified if using common.databaseSecret.name in the helm chart.
• After adding this parameter, the containers start as expected.

What changes were proposed in this pull request?

• Simple documentation / comments about the passwordPath param, within the values.yaml file.
• Alternatively, the comments could be made in the helm chart README - I would be happy to move the info there if preferred.

Check all the applicable boxes

• I updated the documentation accordingly.
• All new and existing tests passed.
• All commits are signed-off.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 56.65%. Comparing base (b192559) to head (1f45ec8).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6895      +/-   ##
==========================================
+ Coverage   56.04%   56.65%   +0.60%     
==========================================
  Files         931      931              
  Lines       71217    66644    -4573     
==========================================
- Hits        39914    37755    -2159     
+ Misses      28249    25835    -2414     
  Partials     3054     3054              

Flag	Coverage Δ
unittests-datacatalog	53.51% <ø> (+1.27%)	⬆️
unittests-flyteadmin	53.10% <ø> (+1.21%)	⬆️
unittests-flytecopilot	41.27% <ø> (ø)
unittests-flytectl	63.53% <ø> (+0.05%)	⬆️
unittests-flyteidl	75.71% <ø> (+2.49%)	⬆️
unittests-flyteplugins	59.33% <ø> (ø)
unittests-flytepropeller	53.37% <ø> (ø)
unittests-flytestdlib	61.73% <ø> (ø)

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[welcome]

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