docs: cite the manifest-next-to-a-path-node rule

The comment on examples/dora-node.yml read as an apology for a workaround.
It is the documented mechanism: a manifest describes exactly one node, and one
sitting next to a plain `path:` node is picked up automatically, which is how a
node gets contract validation without being published.

examples/dora-node.yml

@@ -1,10 +1,14 @@
 # Manifest for the example source node.
 #
-# Its reason to exist is as much mechanical as documentary: `dora validate`
-# matches a node manifest to a `path:` node by entrypoint, and without a
-# manifest here the repository-root dora-node.yml -- whose entrypoint is
-# `dora-image-view` -- gets tried against `examples/source.py`, does not match,
-# and warns. Under `strict_types: true` that warning fails the run.
+# A manifest describes exactly one node, so a second node in this repository
+# gets a second manifest. This is the documented way to type a node you have
+# not published: "a dora-node.yml sitting next to a plain `path:` node is
+# picked up automatically" -- https://dora-rs.ai/dora/hub/node-manifest
+#
+# It is not optional here. `dora validate` matches a manifest to a `path:` node
+# by entrypoint; with no manifest beside examples/source.py the repository-root
+# one is tried against it, does not match its `dora-image-view` entrypoint, and
+# warns -- which `strict_types: true` turns into a failure.
 #
 # Declaring the types here rather than as `output_types:` in dataflow.yml is
 # what makes them checked: an inline annotation shadows the manifest, so the
@@ -50,7 +54,6 @@ outputs:
       Depth of the same scene in millimetres, [height, width]: a flat wall with
       the square standing in front of it.
 
-
 env:
   SOURCE_WIDTH:
     type: int