Secret detection vulnerabilities not auto set to "No longer detected"

What does this MR do and why?

This MR adds some explanation on why secret detection vulnerabilities are not automatically set to "No longer detected", when secrets are eliminated from the scanned files.

This was originally added by this MR. !145442 (merged)

However, this statement was (probably unintentionally) removed when the documentation was split into other two documentations by this MR. !142073 (diffs)

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