Document security MR widget limit

What does this MR do and why?

The Security MR Widget has a limit of how many findings it displays.

This MR documents this limit.

Technical reasons for this limit is captured in #344467 (comment 1449379626)

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.