feat: document refresh rate

Signed-off-by: Anna Larch <anna@nextcloud.com>

admin_manual/groupware/calendar.rst

@@ -10,6 +10,7 @@ You can globally disable sending invitation emails for events, generating the bu
 calendar, and sending email notifications about upcoming events.
 
 .. figure:: images/settings_calendar-server.png
+  :scale: 60%
 
 .. versionadded:: 30 The section will be hidden if no app makes use of the CalDAV backend.
 
@@ -116,9 +117,10 @@ Refresh rate
 ~~~~~~~~~~~~
 
 Calendar subscriptions are cached on server and refreshed periodically.
-The default refresh rate is one week, unless the subscription itself tells otherwise.
+If the calendar server provides a `refresh interval <https://icalendar.org/New-Properties-for-iCalendar-RFC-7986/5-7-refresh-interval-property.html>`_, it is respected.
+Otherwise the default refresh rate is one week.
 
-To set up a different default refresh rate, change the ``calendarSubscriptionRefreshRate`` option::
+To set up a different default refresh rate for calendars without server side refrsh rates, change the ``calendarSubscriptionRefreshRate`` option::
 
  php occ config:app:set dav calendarSubscriptionRefreshRate --value "P1D"