docs: clarify environment variable and system service relationship for config

[DavSanchez]

Please follow conventional commit standards in your commit messages and pull request title.

Give us some context

• What problems does this PR solve? Clarifies how setting configuration options for the infrastructure-agent with environment variables work with system services.
• Add any context that will help us review your changes such as testing notes, links to related docs, screenshots, etc. https://newrelic.slack.com/archives/C5A2QGLKT/p1731946333684059

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