improvement password provider markdown

[JiHyeongSeo]

Description

Nowadays, using druid's password provider, I have experienced a lack of documentation. At least this change will not confuse other developers.

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This PR has:

• been self-reviewed.
  • using the concurrency checklist (Remove this item if the PR doesn't have any relation to concurrency.)
• added documentation for new or modified features or behaviors.
• added Javadocs for most classes and all non-trivial methods. Linked related entities via Javadoc links.
• added or updated version, license, or notice information in licenses.yaml
• added comments explaining the "why" and the intent of the code wherever would not be obvious for an unfamiliar reader.
• added unit tests or modified existing tests to cover new code paths.
• added integration tests.
• been tested in a test Druid cluster.

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[maytasm]

password provider can also be use in other places outside of the config files too. For example, the ingestionSpec. I think it's irrelevant to mention ...metadata.storage... stuff here