Doc: document calendar.caldav.sched.enabled in main Lightning setup doc

git-svn-id: http://svn.code.sf.net/p/davmail/code/trunk@2180 3d1905a2-6b24-0410-a738-b14d5a86fcbd

src/site/xdoc/thunderbirdcalendarsetup.xml

@@ -54,6 +54,10 @@
                     <img src="images/thunderbirdCalendar5.png" alt=""/>
                 </div>
 
+                <p>Lightning mail notifications are broken, activate Caldav notifications to outbox instead.
+                    Make sure calendar.caldav.sched.enabled is true in Thunderbird advanced settings.
+                </p>
+
                 <p>You may want to enable the (experimental) Lightning calendar cache feature to avoid
                     the possibly long calendar load task delay on Thunderbird startup: double click
                     on the calendar in Lightning left pane and activate cache. Restart Thunderbird.