Document support for multiple Chainlink Node secrets files

[ChrisWozniak]

Closing issues

closes #1505

Description

Documents the current Chainlink Node CLI behavior for multiple secrets files: distinct secret fields are merged, while duplicate secret fields are rejected.

Changes

• Replace the incorrect statement that multiple secrets files are invalid.
• Explain the duplicate-secret-field restriction.
• Show repeated -secrets flags in the Docker command.

Validation

• Passed the project Prettier check for the changed MDX file.
• Passed focused [Bug]: <Possible to use multiple secret files from v2.4.0 onwards> #1505 documentation regression checks.
• Passed Git whitespace validation.
• Repository-wide format and lint commands were executed, but fail on existing Windows CRLF and style drift outside this change.
• The link checker could not run on this Windows host because its project script requires Unix nohup and a Vercel production build; Docker is unavailable.

CI status

• The Vercel preview status cannot start because the Chainlink Labs Vercel integration requires organization-side GitHub authorization. This is an integration-access failure, not a reported source build or test failure.

[vercel]

@chris-wozniak is attempting to deploy a commit to the Chainlink Labs Team on Vercel.

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