docs: clarify potential flag collision

[cruwe]

Description

First, thank you very much for developing the trivy suite and making it available to the general public.

More specifically, accessGlobalSecretsAndServiceAccount may not be true for privateRegistryScanSecretsNames to be considered.

Checklist

• I've read the guidelines for contributing to this repository.
• I've added tests that prove my fix is effective or that my feature works.
• I've updated the documentation with the relevant information (if needed).
• I've added usage information (if the PR introduces new options)
• I've included a "before" and "after" example to the description (if the PR is a user interface change).

[CLAassistant]

All committers have signed the CLA.