Update Group Security Dashboard documentation to describe which data are shown

In https://docs.gitlab.com/ee/user/group/security_dashboard/#viewing-the-vulnerabilities we describe the vulnerability chart and the vulnerability list, but we don't give enough information on the criteria used to select vulnerabilities.

We should mention:

1. how numbers in the vulnerability chart are selected
2. how vulnerabilities in the list are selected

This is needed because they are not using the same criteria, and users may be confused by the different numbers in the different places.