Revised revised groups description

[markw]

Revised text for 'What are groups?' box at demo.ckan.org/group (after discussion with IB re #2812):

What are groups?

Groups allow you to group users and data together so that they are easier to manage. Group owners can assign roles and authorisations, giving each project or department control of its own data publishing.

Users can browse or facet by groups, which could be an organisation (for example, the Department of Health) or topic (e.g. Transport, Health), making it easier to find the data they are looking for.