Improve doc for logLevel option

[lym953]

Motivation

Our doc for logLevel option is causing confusion to some users, making them think it can control which user logs are forwarded to Datadog, as shown in https://github.com/DataDog/serverless-plugin-datadog/issues/541

What does this PR do?

Improve the doc to clarify what this option does and doesn't do.

Testing Guidelines

Check the rendered README.md. Looks fine.

Additional Notes

Thanks @astuyve and @purple4reina for explaining that this option does.

Types of changes

• [ ] Bug fix
• [ ] New feature
• [ ] Breaking change
• [x] Misc (docs, refactoring, dependency upgrade, etc.)

Check all that apply

• [x] This PR's description is comprehensive
• [ ] This PR contains breaking changes that are documented in the description
• [ ] This PR introduces new APIs or parameters that are documented and unlikely to change in the foreseeable future
• [ ] This PR impacts documentation, and it has been updated (or a ticket has been logged)
• [ ] This PR's changes are covered by the automated tests
• [ ] This PR collects user input/sensitive content into Datadog