Add documentation for multiple vulnerability selection

What does this MR do?

Adds some documentation around the multiple selection feature introduced in #35816 (closed)

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

Closes #42769 (closed)