Doc updates for SAST/SD rule overrides

What does this MR do and why?

Doc updates for SAST & Secret-Detection Custom Rules - Modifying Pre-existing Rules - Overriding Properties

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.

Related to #235359 (closed)