docs: state what the signing identity does not cover

[dash14]

Summary

The Sigstore identity covers the git ref a release was built from, and the docs described its reach more broadly than that. Two things sit outside it and were not stated: an engine-suffixed Docker tag can be repointed at another engine's image from the same release, and a floating pin accepts any release in its major line. The verification limitation that came closest said a repointed tag could only reach "another image from that same release", which is not what a floating pin allows.

Changes

• The reference-type table is followed by what the identity does not cover, and by which of the two a commit-SHA pin closes.
• The tag-lookup limitation says what a repointed tag can actually reach, and links to that section.

No behavioural change; this describes what verification does today.

[dash14]

Superseded. The engine half of this is no longer accurate: the setup action now reads the proxy engine off the verified image, so a tag pointed at another engine's image from the same release fails rather than being documented as a limitation. The floating-tag half is still true and landed with that change, alongside the engine assertion in the verification flow.