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

LogicalTypes.md

@@ -654,13 +654,18 @@ 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
-   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.
+2. If the repeated field is a group (without `LIST` or `MAP` annotation) with
+   multiple fields, then its type (i.e. a multi-field Tuple) is the element
+   type and elements are required.
+3. If the repeated field is a group (without `LIST` or `MAP` annotation) with
+   one field (with `required` or `optional` repetition) 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. Note
+   that the element type resolves to a single-field Tuple type.
 4. Otherwise, the repeated field's type is the element type with the repeated
-   field's repetition.
+   field's repetition and elements are required. Note that the element type
+   resolves to List or Map type if the repeated field is `LIST`-annotated or
+   `MAP`-annotated; otherwise the element type resolves to a Tuple type.
 
 Examples that can be interpreted using these rules:
 
@@ -691,6 +696,14 @@ 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