feat(adminmanual): Document system address book basics

Signed-off-by: Christoph Wurst <christoph@winzerhof-wurst.at>

admin_manual/configuration_server/occ_command.rst

@@ -515,8 +515,14 @@ bernie::
 
  sudo -u www-data php occ dav:sync-birthday-calendar bernie
 
-``dav:sync-system-addressbook`` synchronizes all users to the system
-addressbook::
+
+.. _occ-dav-sync-system-address-book:
+
+Sync system address book
+^^^^^^^^^^^^^^^^^^^^^^^^
+
+``dav:sync-system-addressbook`` synchronizes all users to the :ref:`system
+address book<system-address-book>`::
 
  sudo -u www-data php occ dav:sync-system-addressbook
 

admin_manual/groupware/contacts.rst

@@ -0,0 +1,19 @@
+==================
+Contacts / CardDAV
+==================
+
+Nextcloud ships a CardDAV backend for users to store and share their address books and contacts.
+
+.. _system-address-book:
+
+System Address Book
+-------------------
+
+Nextcloud maintains a read-only address book containing contact information of all users of the instance.
+
+Disabled users are removed from this address book.
+
+Address Book Sync
+^^^^^^^^^^^^^^^^^
+
+The address book is updated automatically with every added, modified, disabled or removed user. Admins can also trigger a full rewrite of the address book :ref:`with occ<occ-dav-sync-system-address-book>`.

admin_manual/groupware/index.rst

@@ -7,4 +7,5 @@ Groupware
     :maxdepth: 2
 
     calendar
+    contacts
     mail