TaskList: Document relative path behavior

Change-Id: Idda420cfbd2fa701e45e0072730d83d6b8735232
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@theqtcompany.com>
Reviewed-by: Tobias Hunger <tobias.hunger@theqtcompany.com>

doc/src/howto/creator-task-lists.qdoc

@@ -93,4 +93,6 @@
     Lines starting with the hash mark character (#) in the first column are
     ignored.
 
+    If the file path is given as a relative path, it is based on the parent
+    directory of the task list file.
 */