add more documentation about `showanswer` policy key

doc/public/course_data_formats/course_xml.rst

@@ -387,7 +387,14 @@ Inherited
   When this content should be shown to students.  Note that anyone with staff access to the course will always see everything.
 
 `showanswer`
-  When to show answer. For 'attempted', will show answer after first attempt. Values: never, attempted, answered, closed. Default: closed. Optional.
+  When to show answer. Values: never, attempted, answered, closed. Default: closed. Optional.
+    - `never`: never show answer
+    - `attempted`: show answer after first attempt
+    - `answered` : this is slightly different from `attempted` -- resetting the problems makes "done" False, but leaves attempts unchanged.
+    - `closed` : show answer after problem is closed, ie due date is past, or maximum attempts exceeded.
+    - `finished` : show answer after problem closed, or is correctly answered.
+    - `past_due` : show answer after problem due date is past.
+    - `always` : always allow answer to be shown.
 
 `graded`
   Whether this section will count towards the students grade. "true" or "false". Defaults to "false".