docs(index): add config-help keyword aliases for CLASSIFICATION

The CLASSIFICATION schema (docs/function-classification-schema.md) had
no path into config-help's lookup: no fish-config.index keyword, and
the only in-pipeline section (the C1 doc's "For function authors")
doesn't contain the word classification itself, so even the
normalized-heading-scan fallback missed it on that term. Two aliases
added, pointing at the existing section -- no heading renamed, matching
the index file's own stated purpose.

docs/fish-config.index

@@ -291,6 +291,8 @@ network-shadow=### network
 monitor=### monitor
 shell-tools=### shell-tools
 dev-tools=### dev-tools
+classification=### For function authors
+shadow-classification=### For function authors
 c2=## C2 — Startup Side-Effects
 autoexec=## C2 — Startup Side-Effects
 startup=## C2 — Startup Side-Effects