Clarify v0.9.0 import and collection migration guidance

docs/releases/v0.9.0.md

@@ -53,7 +53,8 @@ migration guidance below before upgrading.
 This pre-v1 minor release intentionally changes the v0.8.0 contracts:
 
 1. **Migrate import requiredness.** The YAML `optional` field is removed and is
-   rejected. Replace `optional: true` with `required: false` or omit it; replace
+   rejected. Replace `optional: true` with `required: false`; omission is equivalent
+   when the application does not default imports to required. Replace
    `optional: false` with `required: true`. Imports that previously relied on
    implicit requiredness now need `required: true`, or an application
    `ImportDefaults.Required` default set to true. Review discovery filters:
@@ -78,6 +79,8 @@ This pre-v1 minor release intentionally changes the v0.8.0 contracts:
    defaults to upsert across layers and inheritance when no explicit mode
    applies, rather than unconditional whole-sequence replacement. Declare
    `mode: replace` at the relevant destination where replacement is intended.
+   With append or upsert, `[]` contributes no records and does not clear the
+   collection; null is invalid. Use `mode: replace` with `[]` to clear it.
    An explicit mode persists at its destination until changed; a parent's mode
    does not transfer to a different inheriting destination. Unidentified
    sequences retain strict peer conflicts and whole-value precedence