GH-465: Clarify backward-compatibility rules on LIST type

LogicalTypes.md

@@ -654,13 +654,16 @@ should always be determined by the following rules:
 
 1. If the repeated field is not a group, then its type is the element type and
    elements are required.
-2. If the repeated field is a group with multiple fields, then its type is the
+2. If the repeated field is a group with `LIST` or `MAP` annotation, then the
+   element type is a List or Map type and elements are required.
+3. If the repeated field is a group with multiple fields, then the element type
+   is a multi-field Tuple type and elements are required.
+4. If the repeated field is a group with one `required` or `optional` field,
+   and is named either `array` or uses the `LIST`-annotated group's name with
+   `_tuple` appended, then the repeated type (a single-field Tuple type) is the
    element type and elements are required.
-3. If the repeated field is a group with one field and is named either `array`
-   or uses the `LIST`-annotated group's name with `_tuple` appended then the
-   repeated type is the element type and elements are required.
-4. Otherwise, the repeated field's type is the element type with the repeated
-   field's repetition.
+5. Otherwise, the repeated field's type is the element type with the repeated
+   field's repetition and elements are required.
 
 Examples that can be interpreted using these rules:
 
@@ -691,6 +694,14 @@ optional group my_list (LIST) {
     required binary str (STRING);
   };
 }
+
+// List<List<Integer>> (outer list is nullable with non-null elements,
+// inner list is non-null with non-null elements)
+optional group my_list (LIST) {
+  repeated group array (LIST) {
+    repeated int32 array;
+  }
+}
 ```
 
 ### Maps