VaAvatarGroup component - update docs

[asvae]

Vuestic-ui version: 1.8.0

This is not the most used component (by far), but maybe it deserves some quality.

I added descriptions to storybook https://ui-storybook-develop.vuestic.dev/?path=/docs/vaavatargroup--docs.

So:

• we need to fix stories: you can check how that max prop should work generally. fixed in fix(va-avatar-group): render avatar group with rest count based on max prop #3891
• next - we need to all props and behavior information to docs. Probably we can add that to the same page (va-avatar-group is documented only in context of va-avatar page).