Fix "Group management" documentation

[federicaagostini]

From an email thread with "Setting up new groups and roles for new physics groups" subject, it appears that the Group Management documentation page should be improved.

In particular

• highlight how a new children group can be created
• fix typos
  • childrens --> children
  • children group --> child group (2x)
  • children groups --> child groups (2x)
• rename "child group" into "subgroup"