docs: fix stale digest-match claim in the screenshot mock Dockerfile

The comment said the mock's pin matches the webhook-proxy add-ons, which
this PR's digest bump makes false: webhook-proxy is excluded from the
managed rule and stays behind until its own promotion.

Author: Sergey

tests/haos_image_build/screenshot_engine_mock/Dockerfile

@@ -5,9 +5,13 @@
 # (a bad token fails, exactly like the real engine reaching only the login
 # screen). python:3.13-slim is the same fast, reliable base the dev +
 # webhook-proxy add-ons build from in this bake. No apt, no npm, no Chromium.
-# Digest-pinned (matches the webhook-proxy addon) so the in-Supervisor build is
-# reproducible and Renovate-bumpable — an unpinned tag would reintroduce the
-# floating-base bake failure this change exists to eliminate.
+# Digest-pinned (tracks the coordinated Python base every managed Dockerfile
+# shares -- see the "python" packageRule in renovate.json) so the
+# in-Supervisor build is reproducible and Renovate-bumpable. The
+# webhook-proxy add-ons are excluded from that automation and update only
+# through their own dev-first promotion workflow, so their pin can lag this
+# one. An unpinned tag here would reintroduce the floating-base bake failure
+# this change exists to eliminate.
 FROM python:3.13-slim@sha256:7e3a6aca9d74f93cca21a91d86a8dad8c34749afd5b4a98ee481c9c47b9f5ed4
 COPY server.py /server.py
 CMD ["python", "/server.py"]