Document master-vs-release drift as a fourth unfixable sub-cause

Our haddocks are built from cardano-api master against pinned upstream
releases, but the upstream doc sites we link to publish from their own
master. Symbols moved or removed between the pinned release and current
master surface as 404s through no fault of this script. Pointed out by
palas in PR review.

Author: Jimbo4350

scripts/fix-haddock-links.sh

@@ -91,6 +91,13 @@
 #       c. Haddock-emitted absolute Hackage URLs that lack a package
 #          version (Hackage's routing requires one). A handful, out
 #          of our control, treated as noise.
+#       d. Master-vs-release drift between us and our dependencies.
+#          Our haddocks are built from cardano-api master, which pins
+#          specific releases of upstream packages. The linked upstream
+#          doc sites publish from their master, which may have moved
+#          or removed symbols since the release we pin. Our hrefs are
+#          correct against the pinned release but 404 against current
+#          master haddocks.
 #
 # Escape hatch: FIX_HADDOCK_LINKS_ALLOW_DEAD=1 exits 0 even if there
 # are actionable entries, e.g. to deploy while investigating. Under