Update docs for CE Security Configuration page

What does this MR do?

This is a docs-only update for the CE Security Configuration page.

Originally !53719 (closed) was going to include these docs updates, but it was decided not to enable the feature flag by default just yet.

So, this MR is simply the docs changes from !53719 (closed), but with mentions of enabled by default changed to disabled by default.

Screenshots (strongly suggested)

n/a

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• Documentation (if required)
• Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Availability and Testing

• [-] Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Related to #294076 (closed)