docs for ad-hoc groups and LDAP groups

Copied the "Groups" section from admin-guide/interface.rst to
user-guide/interface/groups.rst. Replaced it with a description of the
extra functionality available to administrators.

Change-Id: Icef3f1cfc09b4ebc0122f68d6c43dd92601bd5a3
Merge "docs for ad-hoc groups and LDAP groups" into develop