feat: document unsharing and resharing

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

Author: miaulalala

user_manual/groupware/calendar.rst

@@ -81,7 +81,7 @@ Sharing calendars
 
 You may share your calendar with other users or groups. Calendars may be shared with write access or read-only. When sharing a calendar with write access, users with whom the calendar is shared will be able to create new events into the calendar as well as edit and delete existing ones.
 
-.. note:: Calendar shares currently cannot be accepted or rejected. If you want to stop having a calendar that someone shared with you, you can click on the 3-dot menu next to the calendar in the calendar list and click on "Unshare from me".
+.. note:: Calendar shares currently cannot be accepted or rejected. If you want to stop having a calendar that someone shared with you, you can click on the 3-dot menu next to the calendar in the calendar list and click on "Unshare from me". To restore a share, the calendar can be reshared again, either for the whole group, resetting all unshares, or for a single user.
 
 Publishing a calendar
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