Document --notes-start-tag requirement in release instructions (#4275)

Author: jlowin

CLAUDE.md

@@ -84,9 +84,11 @@ Only cut releases when the maintainer explicitly asks. Tags follow `v<version>`
 Write the maintainer-approved handwritten notes to a temporary file, then create the release. `--generate-notes` appends the auto-generated changelog after the handwritten content.
 
 ```bash
-gh release create v3.2.0 --target main --title "v3.2.0: Theme Here" --generate-notes --notes-file /tmp/release-notes.md
+gh release create v3.2.0 --target main --title "v3.2.0: Theme Here" --generate-notes --notes-start-tag v3.1.1 --notes-file /tmp/release-notes.md
 ```
 
+**Always pass `--notes-start-tag <last-stable-tag>`.** Without it, `--generate-notes` picks the most recent prior tag as the changelog start point — and if a prerelease exists (e.g. `v3.4.0b1`), it starts from *that*, silently truncating the PR list to only the commits since the beta. Pin it to the last stable release (e.g. `v3.3.1` when cutting `v3.4.0`). Verify after: the compare link at the bottom of the generated notes should read `v<last-stable>...v<new>`.
+
 Most releases target `main`, but maintenance or backport releases may target a different branch (e.g., `release/2.x`). Confirm the target with the maintainer if there's any ambiguity.
 
 The handwritten notes are prepended above the auto-generated changelog and are the part that matters. Do not include a title in the notes body — the release title (`v{version}: {pun}`) already serves as the heading. Work with the maintainer to draft the notes — propose a draft, get feedback, iterate. Do not publish without the maintainer's sign-off.