notify flows: GET returns a schema of the events, fields, node kinds and limits

docs/swarm/notify-router.md

@@ -89,6 +89,22 @@ document, and gets 409 when the stored revision is another. Swarm UI's
 [`/notifications`](ui.md#notification-flows) page shows the document and
 switches built-ins.
 
+**The schema.** `GET /api/notify/flows` also returns a `schema` object, built
+from the constants the check reads, so an editor offers only what a `PUT`
+accepts:
+
+- `source_events`: `[{name, description}]`, the source events.
+- `fields`: `[{name, description}]`, the condition and recipient fields, and
+  the `{field}` placeholders.
+- `node_kinds`: `[{kind, description, fields}]` for `source`, `match` and
+  `todo`; each field is `{name, description, required, values}`, and `values`
+  lists the accepted `{name, description}` pairs (the `upsert` and `clear` of a
+  todo's `op`), empty for other fields.
+- `limits`: `{max_nodes, max_depth, max_flows}`.
+
+A controller that predates `schema` leaves it out of the answer; a client reads
+it as optional.
+
 **What the router runs.** It reads the key at most every 10 seconds, when an
 event arrives, and runs the built-ins you left on, then the custom flows that
 are on and pass the tree check. It logs a custom flow that fails the check,