Clarify current state of SEP and secret detection

What does this MR do and why?

Validate docs are accurately capturing the latest state.

See thread in Slack (internal, 90 days).

Note: I'm basing my understanding of the behavior on discussion in #420332 (comment 1540109429).

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.