docs: github changeloger needs tags to be pushed

refs https://github.com/orgs/goreleaser/discussions/4580

www/docs/customization/changelog.md

@@ -127,5 +127,6 @@ changelog:
 
     * The `github-native` changelog does not support `sort` and `filter`.
     * When releasing a [nightly][], `use` will fallback to `git`.
+    * The `github` changelog will only work if both tags exist in GitHub.
 
 [nightly]: ./nightlies.md