feat(user): document sharing calendars with remote users

Signed-off-by: Richard Steinmetz <richard@steinmetz.cloud>

Author: st3iny

user_manual/groupware/calendar.rst

@@ -108,7 +108,8 @@ always be free, regardless of an events' settings.
 Sharing calendars
 ~~~~~~~~~~~~~~~~~
 
-You may share your calendar with other users or groups.
+You may share your calendar with local users, groups or read-only with remote users on federated
+servers.
 
 .. figure:: images/calendar_sharing_1.png