docs: name the package unbound/srv and unbound/ext need, in the API reference

[raspbeguy]

unbound/srv and unbound/ext write uci packages that only the separate unbound-uci-ext package renders, and both declare reload: ["unbound-uci-ext"]. Without it installed, the first write returns 503 init_script_missing.

docs/resources.md already says so, in full, including the error code. The API reference did not: both tags existed with no description at all, so someone browsing 48 endpoints in Redoc had nothing to tell them these two need a second install.

tag Unbound / Server   -> Recursive DNS server singleton (`config unbound`).
tag Unbound / Srv      -> (no description)
tag Unbound / Ext      -> (no description)

Both now carry one, naming the package and the exact failure, with wording matched to docs/resources.md and the code in transaction.uc rather than paraphrased.

Worth recording that I first reported this as "uapi does not tell users they need it anywhere". That was wrong: my grep truncated the catalog row at 135 characters, and I read a missing phrase as a missing fact. The repo docs were correct all along; the gap was only in the generated reference.

Does this change the wire surface? The document only, and only tag descriptions. No paths, schemas, statuses or headers move.

Was it verified against a real device, or only by CI? Neither is needed for tag prose, but the claim it encodes was measured yesterday on a device: a write to /unbound/ext reaches uci, the unbound-uci-ext init script renders /etc/unbound/unbound_ext.conf, and the managed block appears with the line that was written. make lint, openapi-check and 1234 unit tests pass.

Was LuCI checked? Not applicable; the split between uapi and the renderer package is this project's own arrangement.