Add more information about group assigments

[jaguarfi]

It is possible to have group assignments, but information on how to use these feature may be lacking.

There is one particular use case that may be problematic and needs to be documented. What if a couple of students have submitted assignments by themselves and then they want to form a group. The most reasonable solution is that the teacher goes to the "All submission" page and edit each of the student submissions and add both students as submitters.

Maybe a new issue https://github.com/apluslms/a-plus is required in order to improve the workflow of the teachers who need to reassign the submitters for each submission.