Clarify variant shredding spec

Author: sfc-gh-aixu

VariantShredding.md

@@ -125,8 +125,7 @@ If the value is an array, `value` must be null.
 The list `element` must be a required group.
 The `element` group can contain `value` and `typed_value` fields.
 The element's `value` field stores the element as Variant-encoded `binary` when the `typed_value` is not present or cannot represent it.
-The `typed_value` field may be omitted when not shredding elements as a specific type.
-When `typed_value` is omitted, `value` must be `required`.
+The `typed_value` field may be omitted when not shredding elements as a specific type. The `value` field may be omitted when shredding elements as a specific type. However, at least one of the two fields must be present.
 
 For example, a `tags` Variant may be shredded as a list of strings using the following definition:
 ```
@@ -193,6 +192,7 @@ optional group event (VARIANT) {
 ```
 
 The group for each named field must use repetition level `required`.
+Readers may always assume the group is annotated correctly.
 
 A field's `value` and `typed_value` are set to null (missing) to indicate that the field does not exist in the variant.
 To encode a field that is present with a null value, the `value` must contain a Variant null: basic type 0 (primitive) and physical type 0 (null).