Recurring Tasks - upgrades and bug fixes

- Improved (fixed) logical consistency checks that prevent the
  addition of a recurrence frequency without a due date.
- Improved (fixed) logical consistency checks that prevent the
  addition of an until date with a recurrence frequency.
- When a recurring task is modified, all sibling instances, as well
  as the parent task now get modified.
- When a recurring task is appended, all sibling instances, as well
  as the parent task now get modified.
- Updated documentation.
- It is now possible to upgrade a regular task to a recurring task,
  which is triggered by the "recur" attribute.

html/recur.html

@@ -101,6 +101,13 @@ Permanently delete task? (y/n) y
 This is a recurring task.  Do you want to delete all pending
 recurrences of this same task? (y/n) y</code></pre>
 
+                <h4>Modification</h4>
+                <p>
+                  When a recurring task is modified, all the other recurring task instances will
+                  be modified.  For example, if you raise the priority of one of the recurring
+                  task instances, all will be modified.
+                </p>
+
                 <h4>Recurrence Periods</h4>
                 <p>
                   In the above examples, the recurrence period was specified as "monthly" and