Add note about scan result policies for large groups

What does this MR do and why?

This MR adds information about the fact that it might take a while (up to few hours) to propagate scan result policies created for groups with large amount of projects.

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.