docs(plugin-spec): the migration example no longer teaches the retired onUpgrade hook (#4300)

The page carries a warn Callout stating the onInstall/onEnable/onDisable/
onUninstall/onUpgrade family never existed and was retired (#4212) — and then,
600 lines down, its CHANGELOG example told the reader "the `onUpgrade`
lifecycle hook runs automatically when the package is upgraded to v2.0.0 and
renames the field on existing records." A reader who copies the example writes
a hook that never runs, which is the exact failure #4212 set out to end.

Replaced with the mechanism the same page already documents 320 lines above:
upgrade applies metadata migrations (ADR-0087), and no plugin code runs at
install/uninstall/upgrade.


Claude-Session: https://claude.ai/code/session_01HrRNgrWaRtggzmrHpbomyh

Co-authored-by: Claude <noreply@anthropic.com>

Author: os-zhuang

.changeset/docs-plugin-spec-onupgrade-example.md

@@ -0,0 +1,5 @@
+---
+---
+
+docs: fix the plugin-spec CHANGELOG example that still taught the retired
+`onUpgrade` hook as running automatically (#4212 family). Releases nothing.

content/docs/protocol/kernel/plugin-spec.mdx

@@ -859,8 +859,8 @@ Always include migration guide in CHANGELOG.md for major versions.
 - `account.owner` field (use `account.owner_id` instead)
 
 ### Migration
-The `onUpgrade` lifecycle hook runs automatically when the package is upgraded
-to v2.0.0 and renames the field on existing records.
+Upgrading to v2.0.0 applies the metadata migration that renames the field on
+existing records (ADR-0087). No plugin code runs at upgrade.
 ```
 
 ### 3. Pin Core Dependencies